Online Dating and Problematic Use: A Systematic Review

Despite the constant growth in the use of online dating sites and mobile dating applications, research examining potential problematic use of online dating has remained scarce. Previous research has obviated problematic use of online dating in favour of users’ personality correlates and scams through online dating services. A systematic review was carried out using PsycINFO and Web of Science databases to gather previous findings that address potential problematic use of online dating by (i) identifying use and motivations, (ii) assessing users’ personality correlates, (iii) outlining negative correlates of use, (iv) examining sexual and impulsive behaviour, (v) exploring substance use and behavioural addictions in relation to online dating, and (vi) examining problematic use of online dating, resulting in 43 studies. Findings suggest that personality correlates such as neuroticism, sociability, sensation-seeking, and sexual permissiveness are related to greater use of online dating services. Sex-search and self-esteem enhancement are predictors of problematic use of online dating. Previous research coincides with online dating risks (e.g. fear of deception) and objectification tendency due to online dating services (sites and apps) design. Observations regarding methodological weaknesses and future research implications are included.

users to access online dating anytime and anywhere, making them ubiquitous. Regarding the ubiquity of online dating, Jung et al. (2014) reported that higher availability may be associated with greater engagement in dating apps by showing higher rates of log-ins and use whilst engaged in day-to-day activities.
Greater use of online dating may not necessarily imply the existence of problematic use. However, previous literature in the field of internet disorders has found that extended use (higher frequency of use) is related to higher scores on smartphone addiction (Haug et al. 2015). Yet, extended use is not sufficient to describe problematic use of online dating. Its aetiology and maintenance may be a reflection of diverse factors of different nature (i.e. biological, psychological, and social). Hence, an interdisciplinary explanation (i.e. biopsychosocial framework) is needed. Problematic use of online dating could be explained by utilizing the 'addiction components model' (Griffiths 2005) which postulates all addictive behaviours comprise six core components: (i) salience (dating app use dominates to a great extent the cognitive and behavioural reality of the individual), (ii) mood modification (alteration of mood by use of dating apps), (iii) tolerance (individual's use of dating apps increases over time), (iv) withdrawal (distress when dating app use is interrupted for a longer period of time), (v) conflict (use of dating apps negatively affects the social reality of the user), and (vi) relapse (return to previous patterns of dating app use after interruption).
In terms of structural characteristics of dating applications, location-based structural characteristic appear to facilitate offline encounters (Miles 2017), enabling short-term gratification of users' needs (e.g. users seeking sex encounters are able to find other users at walking distance). In fact, based on the interaction of person-affect-cognition-execution (I-PACE) model (Brand et al. 2016), short-term gratification on dating apps can reinforce the appearance of dysfunctional coping styles to deal with unpleasant emotions (e.g. sadness, frustration and anger) and dysfunctional affective and cognitive responses in relation to dating apps (e.g. craving, urge for mood regulation and attentional bias), which are related to internet-based disorders and exemplify the criteria of Griffiths' (2005) model previously described.
In the scope of internet disorders, and more specifically addiction to social networking sites (SNSs), previous research has reported that availability increases the number of people engaged in the activity, which can lead to excessive use (Kuss and Griffiths 2011). In turn, excessive use of SNSs has been associated with factors such as introversion, extraversion, neuroticism, narcissism and dysfunctional coping mechanisms (Kuss and Griffiths 2011), as well as low self-esteem and anxious attachment (D'Arienzo et al. 2019). In terms of mental health problems, previous literature has noted a positive correlation between depressive symptoms and time spent on SNSs (Pantic 2014), the use of smartphones for different purposes, including SNSs and other media services (e.g. videos and chatrooms) before going to sleep has been found to correlate with depressive symptoms and sleep disturbances in adolescent populations (Lemola et al. 2014). Considering the similarities of SNSs and online dating (sites and applications) and similar findings in online dating research (e.g. low selfesteem related to higher use of online dating, higher availability of online dating sites leading to longer use), it appears plausible to consider previous research investigating SNSs as a guide for online dating research.
Another overlapping phenomenon between SNS use and online dating is the social changes that their usage (SNS use and online dating use) may create in individuals' life. In that sense, Pantic (2014) concluded that SNS use has created changes in how individuals relate to each other in the present time making social interactions more shallow and decreasing communication with family members (Pantic 2014). At the same time, online dating may potentially change the dating scene

Results
This section has been divided into six subsections which cover: (i) usage and motivation, (ii) personality correlates, (iii) negative correlates, (iv) impulsive behaviour, (v) substance use and behavioural addictions, and (vi) problematic use of online dating. Across the subsections, the focus is on the main findings of each study and, when applicable, how these findings relate to overuse/problematic attributes.

Usage and Motivations
A total of eleven studies were found that examined the characteristics of use or motivations of online dating use. Out of the eleven studies, there were ten quantitative studies, all of which were cross-sectional (Corriero and Tong 2016;Gunter 2008;Hance et al. 2018;Houran and Lange 2004;Hwang 2013;Kim et al. 2009;Menkin et al. 2015;Paul 2014;Stinson and Jeske 2016;Valkenburg and Peter 2007), and one qualitative study (Lawson and Leck 2006). One study examined heterosexual respondents only (Hwang 2013), and another study focused on male homosexual populations only (Corriero and Tong 2016), and the remaining studies did not differentiate between sexual orientations.
Before the proliferation of online dating platforms and smartphone applications, Gunter (2008) collected 3844 responses (67% female) from the British population in an online survey available on the website of a research agency that asked questions regarding motivations and users' satisfaction with the online dating service. All age groups were represented evenly: 16-24 years (11%), 25-34 years (31%), 35-44 years (27%), 45-54 years (20%) and 55+ years (11%). Results showed that 29% had used online dating sites and 90% of these users had spent up to £200 over the previous two years using online dating services (Gunter 2008). These  as African-American, 5 as Asian-American, 4 used the term 'other' to describe their racial background, and 7 of the Dating Self-Protection Against Rape Scale (DSPARS); dating background and internet usage questionnaire Greater importance given to self-protective behaviour after reading the online meeting scenario than the face-to-face scenario. This tendency was especially strong among participants who had never been on a date with someone they had met online addiction than non-users. Lower sex addiction scores correlated with social anxiety scores. There was no difference in sensation-seeking between participants with low and high scores of sexual addiction; social anxiety was a major factor affecting the use of internet-dating applications for obtaining sexual partners results were supported by another study  with 367 single respondents (50% females) from the Netherlands. They were asked to complete an online survey that contained a subscale on active intentions from the Dating Anxiety Survey (Calvert et al. 1987). Findings showed that almost half of the respondents (43%) had used the internet to date potential partners. Both studies found differences in terms of use by gender, where men were found to be more likely (40%) to have used online dating services than women (24%) (Gunter 2008). However, there was no difference regarding income or education. Furthermore, in relation to age, it appeared that adults aged between 30 and 50 years were the most active users. In addition to the socio-demographic pattern of use, Hwang (2013) collected data from 2123 heterosexual users' profiles on an American online dating site in Los Angeles and compared the willingness to date between different racial groups (e.g. Asians and Latinos) and within the same group (i.e. whites with whites). In order to do this, demographic measures (i.e. age, gender, marital status, educational level and zip code of residence) were taken; also willingness to date inter-and intra-racially was registered; however, the authors did not specify how they measured that variable (willingness to date inter-and intra-racially). Generally, dating online intra-racially was favoured over inter-racial dating. However, men were found to be higher in willingness to date inter-racially in comparison to women. Nonetheless, considering the specificity of the sample, these results cannot be extrapolated to the general population. Further studies should consider including variability in terms of sexual orientations and cultural background to see if these findings can be replicated.
Considering the expectations of use in terms of finding a perfect partner, Houran and Lange (2004) studied a sample of 222 non-married participants from a paid survey panel (mean age = 37.39 years) and reported that online dating users did not hold unrealistic expectations (i.e. positive distortions towards finding the perfect match). However, the authors did not consider the participants' goals for using online dating, and arguably, depending on users' goals, expectations may differ. Taken together, the previous four studies indicate that young adult men are the most active online dating users tending to date intra-racially. However, three of these studies (i.e. Gunter 2008; Houran and Lange 2004;Valkenburg and Peter 2007) were carried out before the launch of smartphone dating apps, the appearance of which could have resulted in different findings.
Regarding psychological characteristics of users, Kim et al. (2009) surveyed 3354 American respondents across a wide age range (19 to 89 years) and found that those who experienced less dating anxiety were notably more present on online dating platforms. Furthermore, they found that users high in social skills (i.e. sociability), together with high self-esteem, and high relationship involvement were more likely to use online dating services in comparison to those with high sociability and high relationship involvement but with low self-esteem. On the contrary, individuals with low selfesteem and low relationship involvement (together with high sociability) were found to be more active users compared to less sociable participants, and those whose self-esteem was high but who scored low on relationship involvement, or vice versa. To clarify, the effect was only found in the interaction between self-esteem and relationship involvement among those high in sociability. Seemingly, being sociable appears as an important predictor of higher online dating use. However, being highly sociable is not a reliable predictor of online dating use by itself, but only in interaction with individuals' goals and self-esteem. In contrast to these results, a small survey by Stinson and Jeske (2016) of 162 participants found that peer pressure influenced the decision to use online dating services instead of personality factors (e.g. sociability, introversion). The authors claimed that it may be due to the spreading popularity of online dating that personality features were not as predictive in regard to usage tendency.
In terms of individuals' motives, there appear to be many possibilities as to why people date online. In a study of 5434 respondents, Menkin et al. (2015) found that participants generally emphasised interpersonal communication over sex appeal, with women placing greater importance on social interaction, whereas men considered sexual attraction more important than women across all ages. However, younger individuals, aged between 20 and 39 years, considered sexual attraction more important than older individuals (75+ years old). Emphasising sexual attraction, in a study with 62 young men using an all-male dating app (mean age = 22.18 years), Corriero and Tong (2016) identified that casual sex goals were related with desire for uncertainty. Conversely, if users were concerned about their own personal information, health and privacy, then their desire for uncertainty decreased. Therefore, it may be argued that those young users who are looking for casual sex encounters put themselves at higher risk than those who are not looking for sex. This hypothesis is discussed in a later section.
In more general terms, online daters search for companionship, comfort after a life crisis, control over the presentation of oneself to others, to refrain from commitment and societal boundaries, new experiences, and romantic fantasies (Lawson and Leck 2006). In relation to control over self-presentation, it has been claimed that individuals with high rejection-sensitivity tend to feel more comfortable to express themselves in the online medium, and those who feel more comfortable expressing themselves online are found to score higher on online dating use (Hance et al. 2018). One of the reasons for high rejection-sensitive individuals to engage more in the online dating arena may be related to feeling less constrained to show themselves (i.e. 'true self'), identifying less difficulties in the online context. Nonetheless, it appears that common features in online dating like the absence of time limits (i.e. asynchronous communication) and selective self-presentation facilitate deceptive representations of oneself (Hall et al. 2010). In a study of secondary survey data from 4002 US participants, Paul (2014) found that couples who met online had higher split up rates in comparison to partners who met offline. Arguably, typical features of online dating services and apps such as asynchronous communication and selective self-presentation may negatively affect the quality of a long-term relationship between two online daters. Consequently, further studies are needed in the form of longitudinal designs that would help establish the causes that affect the quality of relationships initiated via online dating services.
Overall, the results of this subsection show that the use of online dating platforms is widespread and has grown rapidly in the past few years. In terms of use, younger adult men appear to be the most prevalent users of online dating services. In terms of motivations to use online dating, men favour sex appeal more compared to women. Regarding psychological characteristics, it appears that high sociability and high rejection-sensitivity are associated with higher use of online dating services. The studies reviewed suggest that there are some features in online dating services (i.e. sites and apps) that could enhance the chances of deception and decrease the quality of long-term relationships. Nonetheless, there are some methodological weaknesses (e.g. the use of non-validated psychometric instruments, and non-representative samples) that should be amended in future research so that the internal and external validity of these findings are increased. As to the design, the research should consider longitudinal approaches to help establish the direction of causality (i.e. is relationship quality affected by online dating or are there underlying factors that directly affect relationship quality).

Personality Correlates
Considering the association that exists between specific personality correlates and patterns of use, a total of seven studies (Blackhart et al. 2014;Chan 2017;Chin et al. 2019;Clemens et al. 2015;Hall et al. 2010;Peter and Valkenburg 2007;Sumter and Vandenbosch 2019) were found and reviewed focusing on the association of personality traits and use of online dating services. All the studies assessed used quantitative and cross-sectional methods. Blackhart et al. (2014) surveyed 725 US participants (73.9% females; mean age = 22.31 years) using the Online Dating Inventory (Blackhart et al. 2014) and the Big Five Inventory (John et al. 1991) among other validated scales, and found that individuals low in conscientiousness were more likely to be involved in risky sexual behaviours in the context of online dating. Also, in a survey of 657 Dutch participants (51% females; mean age = 39.26 years), Peter and Valkenburg (2007) found that individuals high in sexual permissiveness and sensation-seeking search more for sex dates. This association was also reported in a study of 257 US heterosexual participants (57.86% males; mean age = 27.14 years) incorporating the integrative model of behavioural prediction, which suggests that intent to engage in a behaviour, normative beliefs, and one's self-efficacy are the key components to predict human behaviour (Fishbein 2000). Findings suggested that those high in sensation-seeking used online dating apps to look for casual partners and romantic dates (Chan 2017). The authors also found associations between trust towards people, sensation-seeking, and higher use of smartphones with increased dating app use, and a direct relationship between smartphone use and dating app use. Arguably, there may be an association between excessive smartphone use and dating app use. Furthermore, Sumter and Vandenbosch (2019) collected data from 171 students of the University of Amsterdam and 370 from a research agency (N = 541; 60.1% females; mean age = 23.71 years) using the Dating App Motivation Scale, based on the Tinder Motivation Scale (Sumter et al. 2017), Dating Anxiety Scale , Brief Sensation Seeking Scale (Hoyle et al. 2002) and Sexual Permissiveness Scale . They reported a positive correlation between sexual permissiveness and dating app use for casual sex dates. The authors also found that the odds ratio for likelihood of being an active user increased by 1.25 for those high in sexual permissiveness. This heightened use was related to feelings of excitement of new activities, coined as the 'thrill of excitement' (Sumter and Vandenbosch 2019, p. 661). Thrill of excitement also works as a motivation for online dating app use for sensation-seeking individuals.
There appears to be agreement concerning the relationship between some personality traits and the motives for online dating use (Sumter and Vandenbosch 2019). In a survey of 678 participants (584 undergraduate students and 94 individuals from the general population from online networking websites; 86% aged between 18 and 20 years), Clemens et al. (2015) took personality measures using the Big-Five Scale (Benet-Martínez and John 1998) and online dating gratifications (i.e. identity, social, companionship, distraction, intercourse, status, and relationship) with blended items from three different validated scales: the General Internet Use Scale (Charney and Greenberg 2002), Television Viewing Motives Scale (Rubin 1981) and Social Networking Scale (Guessennd et al. 2008). Results provided significant correlations between personality traits and online dating gratifications. For example, neuroticism was significantly related to identity gratification, which means that individuals high in neuroticism pursue the creation of their own identity by being free to choose what to show to others. Openness to experience was found to be associated with being social when using online dating sites. Disagreeable individuals were found to use online dating sites to be social and to search for companions. Conversely, those who scored low in disagreeableness were found to use online dating sites with peer pressure (i.e. status). Furthermore, conscientiousness was correlated with finding a romantic relationship. Also, the authors included sex and sexual orientation in the model in order to relate them to personality traits and dating gratifications. Significant associations were found between homosexual participants and gratifications of relationship and sex. Additionally, homosexuals were found to score higher on neuroticism, together with heterosexual women.
It has already been noted that neurotic individuals aim to form their own identity via online dating sites (Clemens et al. 2015). Forming one's own identity on online sites, in this case online dating websites, can lead to misrepresentation (Hall et al. 2010). In a survey of 5020 American online daters (74% females; mean age = 39.8 years), Hall et al. (2010) found that self-monitoring, defined as the quality of adapting one's presentation in order to obtain a desired outcome (Back and Snyder 1988), was a predictor of misrepresentation in online dating. In terms of personality traits, the authors reported that participants low in openness to experience were more likely to misrepresent themselves on online dating sites in order to appear more appealing. Neurotic individuals, who have been claimed to pursue control over their online representation, were not found to misrepresent themselves (Hall et al. 2010).
Regarding attachment styles, Chin et al. (2019) surveyed 183 single American participants, and 60% of those were male (mean age = 29.97 years). A multivariate regression analysis was performed utilising data from the Attachment Style Questionnaire (Simpson et al. 1992), together with some items covering the use of dating apps. Results showed differences in use depending on the type of attachment and reported those with anxious attachment patterns tended to use online dating more than avoidant types.
The results in this section indicate that there is a relationship between the use of dating apps and personality characteristics, such as low conscientiousness, high sensation-seeking, and sexual permissiveness. The relationship suggests that individuals high in sensation-seeking and sexual permissiveness use dating app services for casual sexual encounters. Further research should study the relationship between sensation-seeking and sexual permissiveness with the use of dating apps. Also, there appears to be an association between neuroticism and higher online dating use. However, only two studies have reported a clear positive correlation (Chin et al. 2019;Hance et al. 2018). Regarding the limitations of the studies, all of them were cross-sectional; therefore, no causality or directionality of the findings can be inferred. In terms of samples, there are some limitations regarding generalisability considering that many of the studies used convenience and/or non-randomised samples.

Negative Correlates
This section reviews risks in relation to the use of online dating. A total of ten studies were identified. There were six qualitative studies (Best and Delmege 2012;Couch and Liamputtong 2007;Couch et al. 2012;Erjavec and Fišer 2016;Heino et al. 2010;Vandeweerd et al. 2016) and one paper which contained two studies: one qualitative and one quantitative (Sánchez et al. 2015). Three of the studies were purely quantitative (Cali et al. 2013;Choi et al. 2018;Solis and Wong 2019). Additionally, two studies utilised female-only samples (Cali et al. 2013;Vandeweerd et al. 2016).
According to the studies found in relation to perceived risks, there appears to be agreement on the existence of potential dangers of online dating. Vandeweerd et al. (2016) in an interview-based study with 45 women aged 50 years and older (mean age = 57.3 years) found that there was acknowledgement of risks, such as pervasive lying, attempted financial exploitation, and unwanted electronic sexual aggression (Vandeweerd et al. 2016). Moreover, Solis and Wong (2019) in their study in mainland China with 433 users of dating apps (mean age = 30 years; 57.5% males) reported five categories of perceived risks: lies and deception, sexual risk, dangerous people, self-exposure, and harassment (Solis and Wong 2019). There were some shared perceived risk categories identified by these two studies: lying, finding people with ulterior motives, and aggression. In another study, with a female-only sample from a Midwestern University in the USA (mean age = 24.36 years), Cali et al. (2013) carried out a vignette study comparing two different dating scenarios (i.e. online vs. offline). Participants were randomly assigned to one of the two conditions and were given a description. Following this, they were asked to complete the Dating Self-Protection Against Rape Scale (Moore and Waterman 1999) and some items on internet usage. After analysis, results showed a difference between the two groups. Online dating scenario participants placed more importance on selfprotective behaviours, and those who had never used online dating before scored the highest in self-protective behaviours. Here, it appears that time spent using online dating mitigates the perceptions of risks which could lead to the underestimation of potential dangers. Further research needs to verify this hypothesis. Choi et al. (2018) studied a sample of 666 students from four different universities in Hong Kong (mean age = 20.03 years) and collected data on the use of dating apps and experience of sexual abuse with the subscale of the revised Conflict Tactics Scale (Straus et al. 1996). The data showed that users of dating apps were more likely to have been sexually abused than nonusers in the past year. The use of online dating apps was also associated with lifetime sexual abuse, especially among sexual minorities (i.e. bisexual/homosexual males). These data need to be interpreted cautiously because the data did not discern whether the abuser was met online or offline. Further studies should discriminate whether or not the abuser was met via dating apps.
Among adolescent populations, Sánchez et al. (2015) carried out two studies. The first study was qualitative, with focus groups including 16 participants (eight males) with ages ranging from 14 to 17 years. The focus group data analysis resulted in identifying several factors which were later included in the development of a scale (second study). The scale, namely the Cyberdating Q_A, assesses the quality of online dating among adolescents over six dimensions (online intimacy, emotional communication strategies, cyberdating practices, online control, online jealousy, and online intrusive behaviour). Couch and Liamputtong (2007) interviewed 15 participants from Melbourne (Australia) via online chat, eleven males aged between 24 and 44 years. After carrying out thematic analysis of the transcripts, the main findings reported that participants' management of risks was dependent upon the control they had over their own personal information on the online dating site (e.g. whether they can change their name, not showing telephone number and/or address). In a later study, Couch et al. (2012) carried out a qualitative study with 29 participants from Australia, 12 females, aged between 18 and 70 years (mean age = 32.83). After conducting the interviews via an online chat platform, they found that participants identified risks such as deceit, sexual risks, emotional and physical risks and risks of encountering dangerous and untrustworthy people.
Additionally, one of the key features of online dating (i.e. the screening of multiple profiles in order to select potential partners to establish an interaction which could later lead to an offline date) appears to have counterproductive effects on the users, such as partners' objectification and reduced energies for dating. Heino et al. (2010) reported objectification of the potential dates in a study with 34 American online daters (50% females, mean age = 42) from a large dating site, all of them living in Los Angeles. Participants used many marketplace metaphors when referring to screening profiles, which were themed into five categories: (i) other market's worth, (ii) own market's worth, (iii) shopping for perfect parts, (iv) maximising inventory, and (v) calibrating selectivity (Heino et al. 2010). Another study carried out with 38 older Slovenian adults between 63 and 77 years of age (18 females) found that participants used economic metaphors (e.g. the best of what the market offers, to be back in the market) when speaking about their experience of online dating (Erjavec and Fišer 2016). Similar to these findings, Best and Delmege (2012) in a small-scale study with 15 respondents (66% females aged 18 to 62) from Western Australia found that the use of marketplace metaphors or a 'shopping culture of dating' (Best and Delmege 2012, p. 237) affected the online daters by decreasing their willingness to date. Based on these findings, further research could study the relationship between objectification of others and self in online dating use and mental health problems.
Overall, the studies covered in this section demonstrate that online dating is perceived as more dangerous than traditional offline dating. The perceived risks appear to coincide across studies, mainly involving deception, sexual harassment, and finding untrustworthy people. However, only one study (Choi et al. 2018) identified the risks of being abused in relation to dating apps use, although the findings in this study may be somewhat unspecific because it was not assessed whether the experienced abuse resulted from online or offline sources of aggression. There is agreement on the general perception of risks and the objectification effect by filtering through multiple profiles. Findings come mainly from qualitative studies; therefore, they are informative, but further analysis on more representative populations using quantitative approaches is needed to support these results.

Sexual and Impulsive Behaviour
There is an important body of research studying impulsive behaviours mainly in the form of risky sexual choices in the context of online dating. Consequently, a total of ten studies in relation to online dating were identified examining risky sexual behaviours (Choi et al. 2016a(Choi et al. , 2016bChow et al. 2018;Goedel and Duncan 2016;Heijman et al. 2016;Hospers et al. 2005;Kok et al. 2007;Whitfield et al. 2017), antisocial behaviour (March et al. 2017), and behavioural changes based on site-to-apps shift (Jung et al. 2019). All the studies were quantitative and cross-sectional (Choi et al. 2016a(Choi et al. , 2016bChow et al. 2018;Goedel and Duncan 2016;Heijman et al. 2016;Hospers et al. 2005;Kok et al. 2007;March et al. 2017;Whitfield et al. 2017) with the exception of one longitudinal study (Jung et al. 2019). In terms of samples, six of the studies focused exclusively on men who have sex with men (MSM) (Chow et al. 2018;Goedel and Duncan 2016;Heijman et al. 2016;Hospers et al. 2005;Kok et al. 2007;Whitfield et al. 2017). Choi et al. (2016aChoi et al. ( , 2016b collected data using questionnaires covering the use of dating apps and sexual history, together with some demographic variables. These data were collected in four universities in Hong Kong, which formed a convenience sample of 666 students (mean age = 20.03 years). Of those, at least 296 were male participants (ten did not answer the gender question). The aim was to examine the relationship between smartphone dating apps and risky sexual behaviours (i.e. condomless sex). In the first study (Choi et al. 2016a), results showed a robust positive correlation between dating app use and condomless sex. Additionally, the use of dating apps for a period longer than 12 months was associated with having casual condomless sex in the most recent sexual interaction. In the second study (Choi et al. 2016b), similar results with further associations were found in addition to the previous findings. For example, dating app users and alcohol drinkers were less likely to use a condom during sex (alcohol consumption was categorised as current drinker or non-drinker). Being bisexual, homosexual, or female was significantly correlated with being less likely to have used a condom during the most recent sexual interaction.
Regarding homosexual populations, Chow et al. (2018) studied a large sample of 1672 Australian MSM from the Melbourne Sexual Health Centre (aged between 17 and 78 years; median age = 29 years) in relation to dating apps and use of saliva in sex as a form of lubricant, which has been shown to pose a higher risk of being infected by gonorrhoea (Chow et al. 2016). Findings reported that MSM who used dating apps were 1.78 times more likely to perform rimming (oro-anal sex) and 1.63 times more likely to use saliva as lubricant during anal sex (Chow et al. 2018). In line with these findings, Goedel and Duncan (2016) found a positive correlation between condomless sex and use of several dating apps in a sample of 174 New York City male users (age range 19 to 58; mean age = 30.8 years) of an all-male dating app. Additionally, a significant relationship between alcohol and drug use and condomless sex was found (drugs and alcohol consumption data were collected via an item based on a retrospective account of the last three months in conjunction with dating app use).
In contrast to these findings, Heijman et al. (2016) studied a sample of 3050 MSM Amsterdam participants (mostly Dutch [73.8%] with a median age of 37 years). The results found no significant association with dating app use and condomless sex among HIV-negative users; conversely, HIV-positive users were found to be more likely to perform anal sex without a condom, indicating that there are differences in risky sexual choices by MSM in the context of online dating. However, this association was not significant after inclusion of partnership characteristics in the multivariate model (e.g. HIV status, ethnic origin, and age). The authors suggested that knowing more information about partners (i.e. HIV status, lifestyle concordance, and ethnic origin) works as a mediating effect for condomless sex in the context of online dating.
In a previous study with MSM in the Netherlands, Hospers et al. (2005) reported a higher percentage (39%) of condomless anal sex especially in HIV-positive online daters in comparison to HIV-negative daters, but no differences were found between offline and online samples. Even though the sample comprised 4984 users (mean age = 33.2) of an online dating platform, the results may be interpreted with caution because smartphone dating apps were non-existent at the time the study was published. Nonetheless, a more recent study found no correlation between the use of dating apps and condomless sex among a homosexual sample of 545 men (mean age = 36.81 years) (Whitfield et al. 2017). Nonetheless, Whitfield et al. (2017) found ethnic group differences in terms of condom use in online daters, and the results of their research show that individuals with Latino/Hispanic origin are found to be 0.46 times more likely to have unprotected anal sex than Whites; other ethnic origins such as American Indian, Alaskan, Asian and Hawaiian were categorised as 'other' (Whitfield et al. 2017, p. 780) which increased the chances of condomless anal sex by 0.35 in comparison to their White counterparts in individuals who use online dating.
In order to explain the factors involved in the decision-making of sexual risky behaviours among MSM who actively use online dating platforms, Kok et al. (2007) used the theory of planned behaviour (Ajzen 1991) and found that attitude (e.g. behavioural beliefs about the use of condoms), subjective norms (i.e. normative beliefs), and perceived control (i.e. self-efficacy) explained 55% of the variance in intention of using protection during anal sex. Fantasising about condomless sex was found to have a direct effect on intention to carry out condomless sex (intention is considered by the theory of planned behaviour to be the most reliable predictor of behaviour) (Ajzen 1991;Kok et al. 2007). In relation to online dating apps, it could be argued that specific structural characteristics (e.g. chat, sharing pictures) may increase fantasising about condomless sex. However, further research is needed to relate the aforementioned structural characteristics of dating apps and sexual behaviour.
Regarding behavioural changes among computer online dating and smartphone dating apps, Jung et al. (2019), in a study that accessed data from 100,000 users (geographical location was not specified) of an online dating site (female mean age = 36.10 years; male mean age = 33.22 years) reported that the shift from computer-only access (i.e. online dating site) to smartphone access (i.e. dating app) produced a behavioural change in the users, such as increasing the number of visits to others' profiles, sending more messages, and achieving more matches (Jung et al. 2019). As a consequence of computer-to-smartphone shift, the authors noted that men had increased impulsivity (i.e. they became even less deliberate in terms of quantity of messages sent and their targets). Regarding disinhibition, both men and women lowered their partners' preference standards. For example, viewing profiles of individuals from a different ethnic background increased by 85.3% per week for females and 127% for males (Jung et al. 2019). Therefore, according to these results, there appears to be an effect on the ubiquity factor to becoming more engaged and presumably increasing the chances of developing a misuse pattern of online dating services when using smartphone dating apps rather than computer-based online sites.
According to March et al. (2017), there is a relationship between dysfunctional impulsivity and antisocial behaviours, such as trolling (i.e. the act of being provocative, offensive or threatening [Bishop 2014]) on the Tinder app. In their study with 357 participants from Australia (mean age = 22.50 years), findings suggested that traits of psychopathy, sadism, and impulsivity were positively related to acts of trolling. Taking these two studies together (Jung et al. 2019;March et al. 2017), it appears that impulsivity plays a role in increasing users' behavioural repertoire in the context of online dating and also provides the possibility to engage in non-adjusted behaviours.
Overall, the results presented in this section suggest that online daters have higher chances of behaving impulsively in comparison to non-users in terms of risky sexual choices. The behaviours covered were mostly of sexual nature and focused mainly on homosexual male populations (MSM). This biased focus may be due to the fact that homosexual men's sexual practices pose a higher risk of HIV infection. Nonetheless, it could be beneficial for the sake of generalisability to know if these results can be replicated across individuals with other sexual orientations (i.e. heterosexual, bisexual, homosexual women). Apart from sexually risky behaviours, it has been reported how the ubiquity factor of dating apps facilitates users' engagement (Jung et al. 2019), potentially leading to an addictive pattern of use, but there is a need for further research to support this hypothesis.

Substance Use and Behavioural Addictions
In the final selection of studies, there are only two studies that have examined the relationship between online dating and substance use addiction (Boonchutima and Kongchan 2017;Choi et al. 2017) and one was dedicated to a behavioural addiction (i.e. sex addiction and online dating) (Zlot et al. 2018). Boonchutima and Kongchan (2017) surveyed a sample of 350 MSM from Thailand (three out of four respondents aged 18 to 35 years) and asked about their online dating app use, sexual history, drug use history and intention of using drugs. Regression analysis reported that over 73% of the participants were using dating apps to find partners and to invite others to use illicit drugs with a 77% invitation success rate. Furthermore, one in three substance users (34.3%) engaged in condomless sex. Therefore, according to the findings, there may be an association between illegal drug use and condomless sex. Nevertheless, it should be noted there is no mention regarding what type of illicit drugs was used.
Regarding alcohol consumption and online dating, Choi et al. (2016b) recruited a convenience sample of 666 students from Hong Kong, and correlational analysis found that being an online dater was associated with inconsistent use of condoms during sexual interactions (use of condoms was categorised as consistent if condoms were always used, or non-consistent if condoms were not used every time) and being a current drinker (categories were non-drinker or current drinker, no specific description of those categories are provided), concluding that 'dating apps tend to skew their users toward risky sexual encounters' (Choi et al. 2016b, p. 8). In a later study, Choi et al. (2017), with a convenience sample of 666 students (mean age = 20 years) from Hong Kong, reported a relationship between longer use of online dating (i.e. more than a year) and recreational substance use in conjunction with sex. Again, the specific substances were not mentioned and were coined as recreational drugs (alcohol was independent of the recreational drugs category). It would be useful for further research to specify the respective substances as the scope of illicit or recreational drugs can be extensive. According to these studies, the co-occurrence of substance use with risky sexual behaviour in the context of online dating was indicated. Nonetheless, caution needs to be used with regard to this assumption because the assessed samples were skewed towards MSM; therefore, generalising the results to the general population is not possible.
In relation to behavioural addictions in the context of online dating, Zlot et al. (2018) studied a sample of 279 participants from Israel comprising 128 males (mean age = 25 years). In order to collect data, participants answered a series of validated psychometric instruments that were integrated in an online questionnaire. Measures included the Liebowitz Social Anxiety Scale (Liebowitz 1987), the Sensation Seeking Scale (Zuckerman et al. 1964) and the Sexual Addiction Screening Test (Carnes 1991). Following the analysis, associations were found between users of dating apps and higher scores on sexual addiction measures in comparison to non-app users, as well as a positive correlation between social anxiety and the use of smartphone dating. Again, the relationship between anxiety-tendency factors and the use of online dating was supported as was mentioned in the preceding sections.
The scarcity of the literature limits the conclusions. However, the findings can be considered as a guide for future study examining substance use and other types of behavioural addictions with online dating. There appears to be a relationship between substance use among partners who have met via online dating, at least among MSM who use dating apps. In relation to substance use and online dating among heterosexual populations, data come from only one study that reported no direct relationship (Choi et al. 2017). However, limitations in both studies include the use of general terms such as illicit/recreational drugs which necessitates further specification and replication. In terms of behavioural addiction, only sex addiction has been studied and it was found to be related to dating app use (Zlot et al. 2018).

Problematic Use of Online Dating
To date, only two studies have exclusively focused on problematic online dating. Both studies were quantitative and developed validated psychometric scales (Orosz et al. 2016. One of the studies used a mixed-methods approach . The two studies solely focused on one specific dating app (i.e. Tinder). In the first study, Orosz et al. (2016) developed a psychometric instrument to assess the problematic use of Tinder (Problematic Tinder Use Scale, PTUS). This self-report measure is based on the components model of addiction (Griffiths 2005), which comprises six characteristics of addiction: salience, mood modification, tolerance, withdrawal, conflict and relapse. In order to validate the PTUS, a sample of 430 Hungarian Tinder users (243 females; mean age = 22.53 years) was selected, and the six-item unidimensional structure showed good reliability and factor structure. In the second study, Orosz et al. (2018) carried out three different studies. First, with a sample of 414 Hungarian respondents (246 females; mean age = 22.71 years), the TUMS (Tinder Use Motivations Scale) was developed, resulting in the identification of four main motivations of Tinder use arising from a 16-item first-order factor structure (i.e. sex, love, self-esteem enhancement, and boredom). In the second study, with a convenience sample of 346 participants (165 females; mean age = 22.02), measures were taken from the newly developed TUMS, together with the PTUS, and the Hungarian Big Five Inventory (John and Srivastava 1999). The results were weak in relation to personality factors and the four main motivations for Tinder use. However, self-esteem enhancement was related to Tinder use. In the third study, 298 participants (177 females; mean age = 25.09) were assessed with the TUMS, PTUS, the Hungarian 10-item Rosenberg Self-Esteem Scale (RSES) (Randal et al. 2015;Urbán et al. 2014), and the Hungarian version of the Basic Psychological Need Satisfaction and Need Frustration Scale (BPNSFS) (Chen et al. 2015;Tóth-Király et al. 2018). The results showed that relatedness frustration (i.e. needs not met by affection and care from relevant others) predicted the motivation of self-esteem enhancement which was found to be one of the motivations associated with problematic use of Tinder, together with the sex motive.
Overall, the studies presented in this section are not sufficient in terms of quantity to consider online dating addiction as an entity. However, the studies are of general interest to researchers considering the widespread use of dating apps and provide insight in relation to factors such as self-esteem and sex-searching that may be related to the development of problematic patterns of use. Even though there is a scarcity of literature examining problematic use of online dating, there is some research that appears to support the findings presented in this section. Further study is needed to consider the relevant factors that have been suggested as predictors of problematic use, self-esteem and sex-searching motives, with a cross-cultural approach in order to inform of possible cultural differences in relation to problematic use. Also, other dating apps could be subject of study to examine if there are any differences in terms of motives that could lead to problematic use.

Discussion
The present paper reviewed the literature concerning the use of online dating focusing on problematic online dating (computer-based and smartphone apps), characteristics of users (e.g. personality correlates, users' motivations), and consequences of use (e.g. risks associated with the use of online dating, impulsivity, use of drugs in conjunction to online dating). Due to the lack of previous literature on problematic use of online dating, socio-demographic and psychological characteristics (e.g. gender, age and personality) are informative with regard to which specific individual characteristics relate to greater use of online dating. Even though longer-time use cannot be considered as problematic or addictive per se, it could be a reference point for future research in the field.
In terms of use, two of the reviewed studies pointed out that between 29% and 43% of their samples had used online dating services. However, these studies were published in 2007 and 2008, and in one decade, the usage of online dating platforms (including dating sites and dating apps) has been extended reaching up to 8000 different dating sites in the world, representing a business worth almost US$2 billion per year (Matthews 2018). The growth in this service may be due to different reasons, and as with other forms of internet use (e.g. social media use, online gaming, online shopping, etc.), much of this use may have nothing to do with addictive patterns, but with passing time and being a pleasurable activity.
Nevertheless, online dating developers have acknowledged that design is made to engage the user and increase monetisation of the business (Jung et al. 2014). Even though the design of dating apps has not been studied in the field of addiction, previous literature examining SNS use suggests that user interaction such as scrolling, tapping, and typing is related to smartphone addiction (Noë et al. 2019). Considering that dating apps have a similar user interaction design (i.e. typing, scrolling/swiping, and liking), comparable associations with addictive patterns of use may exist. Further research is needed to confirm such a speculation.
In terms of personality correlates, reviewed studies pointed out that sociability, anxious attachment style, social anxiety, lower conscientiousness, higher sensation-seeking, and sexual permissiveness were associated with higher use of online dating (sexual permissiveness and lower conscientiousness have also been related to sex-searching in the context of online dating) (Blackhart et al. 2014;Chin et al. 2019;Kim et al. 2009;Peter and Valkenburg 2007;Zlot et al. 2018). Likewise, SNS research has suggested that higher extraversion, social anxiety, loneliness, and lower self-efficacy are related to Facebook addiction (Atroszko et al. 2018), higher extraversion and neuroticism to SNSs (Wang et al. 2015), and higher sensationseeking to smartphone addiction (Wang et al. 2018). Neurotic correlates (i.e. social anxiety, neuroticism, and anxious attachment style) of SNS and online dating research have been found, with these characteristics having been associated with higher use, operationalising the definition of neuroticism as being highly anxious, depressed, and low in self-esteem (Eysenck 1965), and it could be argued that some of the motives of use claimed for these individuals could work as a form of avoidance or escapism from distress (e.g. distraction), leading to a negative reinforcement of the behaviour (i.e. online dating) that could heighten the chances of developing any kind of misuse or excessive usage pattern. Furthermore, the relationship between anxiety traits and neuroticism has been upheld by a great body of research in behavioural addictions (Andreassen et al. 2013;Atroszko et al. 2015;Balta et al. 2018;Kuss et al. 2013Kuss et al. , 2014. Therefore, considering this association, it is recommended that future research should study this relationship with the problematic use of online dating. To date, only one study has related self-esteem enhancement to problematic use of Tinder . Considering that anxious attachment, and generally anxiety-tendency correlates (i.e. neuroticism) are associated with lower measures of self-esteem (Lee and Hankin 2009), it could be argued that anxious users find online dating a form of validation, which can serve as positive social reinforcement that can increase the chances of continuing the use of online dating for longer periods of time, and potentially developing addictive-like patterns of use (e.g. craving for the use/validation, salience of use and mood modification).
Another form of problematic use of dating apps, more specifically Tinder, is sex-search use . As previously discussed, sex-search use of online dating has been related to higher measures of sexual permissiveness, sensation-seeking, and lower conscientiousness. Furthermore, in one study, sex addiction was related to greater use of online dating sites (Zlot et al. 2018). Being a homosexual man has also been related to sex-search motives (Clemens et al. 2015), which may explain the bias towards homosexual men samples examining risky sexual behaviours in the context of online dating.
The reviewed studies supported an association between dating app use and condomless sex in comparison to non-dating app users, even though there are some studies that did not find this association (Heijman et al. 2016;Hospers et al. 2005;Whitfield et al. 2017). Nonetheless, homosexual men may be at higher risk of problematic use of online dating due to the prominent sex-search motive for online dating. Finding casual sexual partners in online dating services is facilitated by some apps that show how far users are from each other (i.e. geographical distance). This structural characteristic (GPS-based service) may be related to higher impulsive decisions and problematic use of online dating. Arguably, by showing up walking-distance profiles, it is easier to engage in casual dates and this may serve as a self-esteem enhancement mechanism, as previously discussed, which may increase engagement and usage of online dating services. However, further research is needed to support this association and how the different structural mechanisms of the respective dating apps affect measures of well being in users. Drawing upon chatting via online dating sites and apps (one of the structural characteristics of online dating is the possibility of engaging in online chatting with other users), it may be relevant to consider the act of 'sexting' (the act of sending sexual content or explicit nude pictures or videos via text messages) (Gordon-Messer et al. 2013) as a potential factor for increasing sex-motive search. Previous research has associated sexting with risky sexual behaviour (Klettke et al. 2014) Consequently, chatting (one structural characteristic of dating apps) may facilitate the appearance of sexting, in turn increasing the chances of risky sexual behaviours. Sexting through dating applications may as well increase the sex-search motive of users (i.e. casual sex dates) which has been found to be a predictor of problematic use of dating apps. However, further study is needed to provide evidence in order to relate chatting through dating apps and sexting, and how this may influence the appearance of sexual behaviour (e.g. risky sexual behaviour and/or heightened sex-search motive).
Some of the reviewed studies concerning associated risks converge on the findings that generally online dating users find online dating to have specific risks, including deceit, fear of physical harassment, and financial exploitation. Additionally, there is a body of research that points to the objectifying environment that emerges in online dating (e.g. through using market-like vocabulary and filtering through numerous profiles). It is of concern that objectification of other users may increase self-objectification (Koval et al. 2019), whose mental health consequences have been noted in previous literature including clinical symptoms of depression and eating disorders (Jones and Griffiths 2014;Register et al. 2015). Therefore, further research should study the emotional experience of users and consider how longer time of use may influence wellbeing measures and clinical mental health symptoms through self-objectification.
Regarding methodology, some weaknesses limit the strength of the findings in the reviewed studies. First, cross-sectional design prevents from making causality inferences and to know the directionality of the results (e.g. condomless sex leads to using dating apps or using dating apps leads to having condomless sex). Second, some of the measures present limitations which may bias the results (e.g. use of non-validated items, lack of categorisation, and specificity). Third, some samples limit the external validity of the findings (i.e. convenience samples and specific-population samples). Therefore, it is recommended for further study to (i) use more diverse samples, (ii) consider methodologies that can establish causality, and (iii) collect data using self-reports together with interviews to increase internal validity. In addition to the latter, it could be useful to collect real-life measures of online dating use which assess the temporal stability of usage and may provide some insightful objective data that self-report measures cannot facilitate, such as using the experience sampling method (ESM), which is defined as a research procedure by which participants respond to a series of questions multiple times a day during a specific period of time (Larson and Csikszentmihalyi 2014). All of these proposals would help to overcome the present limitations of these studies and provide more robust insights in the field of online dating utilising the highest standards of empirical research.
This current systematic review presents a number of limitations. First, there are some studies that do not specify whether their findings are based on online dating sites, mobile applications, or both. This is necessary in order to differentiate the distinctive phenomena of each service. Second, online dating services include a great variety of apps and sites; therefore, including all of them under the term online dating services may be reductionist and ignore different processes (i.e. psychological and behavioural) that may arise from their use. Third, due to the paucity in previous research in the field of online dating, some conclusions are based on a limited amount of studies, and further study will be needed in order to support current findings and conclusions. Lastly, considering that the field of online dating research is growing over time, it is likely that studies under the process of submission or publication have been not included in this review.

Conclusions
Online dating has become an extended service across technological societies. The present review is the first attempt to gather empirical findings regarding the use of online dating services (sites and smartphone applications) and problematic use of online dating. Findings in this this review indicate that there are personality correlates such as sociability, sensationseeking, sexual permissiveness, and anxious attachment that correlate to greater use of online dating. Self-esteem enhancement and sex-search motives have been related to problematic use of online dating (more specifically of the dating app Tinder). Other results indicate that users consider online dating as more dangerous than offline (i.e. traditional) dating, as well as more objectifying. Additionally, online dating services facilitate casual encounters (i.e. hook-up dates) which represent a public health concern in terms of sexually transmitted infections (STIs) and substance use (alcohol and recreational drugs).
Funding Information This research is funded by Doctoral Training Alliance (DTA3) in COFUND with European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant.

Compliance with Ethical Standards
Conflict of Interest The authors declare that they do not have any interests that could constitute a real, potential or apparent conflict of interest with respect to their involvement in the publication. The authors also declare that they do not have any financial or other relations (e.g. directorship, consultancy or speaker fee) with companies, trade associations, unions or groups (including civic associations and public interest groups) that may gain or lose financially from the results or conclusions in the study. The first author's university is currently receiving funding from Doctoral Training Alliance under the Marie-Curie program, Horizon 2020 (sources of funding are acknowledged).

Informed Consent Not applicable
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