Unlike student attrition, research on doctoral attrition has not been guided by much theoretical considerations (Tinto, 1993: 231). We suggest that similar to attrition during university studies, attrition at the doctoral stage can be conceived as a process in which doctoral students decide to stay or leave. Tinto (1993), in his seminal work on undergraduate student attrition, argues that students are equipped with a set of pre-entry attributes, such as prior schooling outcomes and family background, which influence their ability to integrate into the academic and social system of a higher education institution, and ultimately their decision to drop out. Institutional experiences depend not only on pre-entry attributes but also on institutional factors, such as the amount of intellectual stimulation, or the security of funding. In addition, factors external to the institution affect the decision to leave, for example, family obligations (Tinto, 1993).
More recently, rational choice theory has been proposed to enhance the understanding of study success at the undergraduate level (Beekhoven et al., 2002). Putting an even stronger emphasis on individual choices than Tinto, this theory assumes that students evaluate the available alternative choices, staying or leaving, by considering the probability of success as well as the costs and benefits associated with these choices. If the subjective “utility” of leaving is higher compared to staying, a student leaves. The basic concepts and predictions of rational choice theory coincide well with Tinto’s general idea. For example, from a rational choice perspective, positive peer-interactions (i.e., social integration in Tinto’s words) increase the subjective benefits of persisting in terms of enjoyment. Similarly, higher performance (i.e., academic integration) increases the probability to successfully complete the degree.
Decisions to leave or continue doctoral training are shaped by the national context in which they occur. Therefore, findings from international studies are not necessarily transferable to the German context, which is characterized by some national specifics. Unlike the US system, where doctoral candidates are selected in a formalized process, German doctoral students are often selected informally by a single professor, who is also the supervisor of the thesis (Hüther & Krücken, 2018). Possibly, these different recruitment processes lead to a rather heterogeneous pool of doctoral candidates in terms of motivations and scholastic abilities, with potential implications for dropout propensity. Moreover, while there is a clear trend towards giving doctoral education more structure, only a minority of German doctoral students attends a US type graduate school with a structured curriculum (Consortium for the National Report on Junior Scholars, 2017). Instead, the vast majority of doctoral students is employed in fixed-term, part-time positions at chairs or in projects of a professor. This implies not only a strong dependence on a single (mostly male) professor, and, in many instances, less support in the form of structured doctoral training and regular feedback by a team of other researchers. Most PhD candidates also have teaching or research obligations, which leaves less time to work on their thesis. Another national specific is that there are hardly any permanent positions below the level of professor, which is typically achieved around the age of 42 (Hüther & Krücken, 2018). Compared to other labor market sectors, employment insecurity on the academic labor market is very high.
Drawing on these theoretical premises, the following sections discuss the empirical evidence on pre-entry, institutional, and external (i.e., non-institutional) factors that may influence the propensity to drop out of doctoral studies, and develop hypotheses to explain these associations. Our selection of variables is guided by theoretical considerations but also by data availability. That is to say, we do not develop hypotheses for variables we cannot measure empirically, such as motivational characteristics or the subjectively perceived probability of success. The theoretical framework of our study is presented in Fig. 1.
A wealth of studies observed gender gaps in educational outcomes, however, whether males and females experience different dropout patterns from doctoral training, is an empirically open question. According to Bair and Haworth’s (2004) review, the majority of studies indicate that gender is not significantly related to doctoral degree completion. This conclusion received confirmation in more recent studies (Mastekaasa, 2005; Wollast et al., 2018; Wright & Cochrane, 2000; Ampaw & Jaeger, 2012). However, other studies, including a recent German study, found that men are (slightly) less likely to drop out (Groenvynck et al., 2013; Castelló et al., 2017; Lörz & Mühleck, 2018; Lott et al., 2009). A reason for these divergent findings may be that studies vary tremendously in their designs and types of samples, with the majority of studies relying on data from single institutions or disciplines. Theoretically, we assume that gender-specific dropout rates might be driven by career perspectives for early stage researchers. Given that the division of labor within partnerships is still gender-specific while academic career progression demands a high degree of work engagement and flexibility, women may perceive or anticipate greater work-family-conflicts, making a career in science less attractive to them (Dubois-Shaik & Fusulier, 2017). Regarding career perspectives, the German higher education system is known to offer particularly insecure employment conditions for early stage researchers due to a lack of tenure track positions (Hüther & Krücken, 2018). Moreover, female scientists may suffer from subtle evaluation bias in favor of male scientists (Moss-Racusin et al., 2012). Such a bias may be particularly strong in the German system in which the dependence on a single supervisor is particularly strong. Accordingly, a Germany-based study by Jaksztat (2017) suggests that female doctorate holders perceived less promotion and support during their PhD phase than male doctorate holders. These disadvantages should shape women’s decisions to further strive for an academic career and they might decrease their motivation and opportunities to complete a PhD. Accordingly, we expect that women in Germany are more likely to drop out than men (H1).
Much less attention has been devoted to the social origin of doctoral students. Booth and Satchell (1995) found no effect for parental social class in a sample of British PhD students in the 1980s. For Germany, Lörz and Schindler (2016) reported that students from lower educated families have slightly lower PhD completion rates, which are primarily caused by differences in academic performance. According to rational choice theory, families try to avoid intergenerational social downward mobility (Breen & Goldthorpe, 1997). This so-called motive of status maintenance implies social costs of dropping out of doctoral education, but only for those students, whose parents hold doctoral degrees themselves. For all other students, withdrawal would not increase the risk of intergenerational downward mobility. Moreover, parents with doctorates may be better able to support their children financially or with information and tacit knowledge, which raises their success probability. Based on these theoretical considerations, we expect that children whose parents hold a doctorate are less likely to drop out than other doctoral students (H2).
Prior academic performance
High levels of prior academic performance are an indicator of high cognitive skills and effort (Schneider & Preckel, 2017), which should raise the probability of successfully completing a PhD. Contrary to this expectation, Bair and Haworth (2004) concluded in their US review that academic performance indicators are not reliable predictors of persistence to the doctoral degree. For example, only five of fifteen studies found master’s Grade Point Average to relate positively to doctoral degree completion. An explanation for these weak findings may be that PhD students are a highly selective group with only little variance in prior performance. However, an indication that this conclusion may not hold true for non-US countries is given in two Belgium studies (Wollast et al., 2018; Visser et al., 2007) and two German studies (Lörz & Mühleck, 2018; Lörz & Schindler, 2016) which found that lower grades are positively associated with dropout. In countries such as Germany, where selection into doctoral studies is less structured and therefore possibly less performance-based, the variance in prior academic performance among PhD candidates may be higher than in the US context. In such a setting, we expect that higher levels of prior academic performance are negatively correlated with the likelihood to drop out (H3).
Dropout rates differ considerably between subject areas (e.g., Groenvynck et al., 2013; Booth & Satchell, 1995; Bair & Haworth, 2004; Wright & Cochrane, 2000; Euler et al., 2018). The general pattern is that students in the humanities and social sciences are less likely to complete their PhD than those in natural sciences and medicine. Theoretically, such a pattern is to be expected: following rational choice theory, completion rates should be high in subjects in which the working conditions are more favorable, or where the expected monetary returns of a PhD are high. Favorable working conditions may be more prevalent in the natural sciences including medicine. Here, research is typically organized in laboratories, where a team of PhD candidates works under direct supervision of an advisor. Such a setting may encourage regular peer exchange and strong supervisory support, even without the structural setting of a graduate school. In the humanities, law, and social sciences, working on a doctoral thesis tends to be less integrated in larger research contexts, possibly leading more frequently to a lack of feedback and feelings of isolation. Moreover, disciplines differ regarding the monetary returns of a PhD. According to a German study by Heineck and Matthes (2012), the returns of a PhD are the highest in law, engineering, natural sciences, and medicine. Accordingly, we expect for Germany higher dropout rates in the humanities and the social sciences than in law, engineering, natural sciences, and medicine (H4).
Funding organizations select their scholars primarily based on previous performance and quality of the suggested PhD proposal (Nünning & Sommer, 2007). Therefore, it can be assumed that scholarship holders are a selective group in terms of ability and motivation. Scholarships for PhDs usually cover costs of living for several years, giving students the leeway to focus exclusively on their research project. In addition, funding organizations typically offer mentoring through experienced researchers, and organize networking activities with other PhD students. These factors should increase the success probabilities of doctoral research. Generally, the empirical literature finds that students with secure funding are more likely to succeed (Bair & Haworth, 2004; Ehrenberg & Mavros, 1995; Stock et al., 2011; Visser et al., 2007; Wollast et al., 2018). However, it is less clear whether scholarships lead to higher completion rates as opposed to other forms of secure funding (Ampaw & Jaeger, 2012). Because scholarship programs commonly comprise further aspects beyond funding (selection processes, institutionalized mentoring, and networking), we expect that scholarship holders are less likely to dropout from doctoral studies than other PhD candidates (H5).
Contact with supervisor
The supervisor-student relationship is of genuine importance for the success of a doctoral research project (e.g. Mainhard et al., 2009; Skopek et al., 2020). As experts in their fields, they provide intellectual support and information, help their students to establish networks to other researchers, and are supportive in case of difficulties. Furthermore, they can serve as role models, give emotional support, and create an enjoyable working environment, leading to higher student satisfaction. It has been shown repeatedly that positive and frequent contact between advisor and student is a crucial aspect of doctoral completion, while a lack of interaction and support increases the dropout propensity (e.g., Golde, 2005; Lovitts, 2001; Bair & Haworth, 2004; Castelló et al., 2017; Ampaw & Jaeger, 2012). Hence, we expect that a strong and positive relationship with a supervisor has a negative effect on doctoral dropout (H6).
Exchange with other PhD students
Another important source for intellectual and emotional support are peers. In some cases, contact with other doctoral students may, in part, compensate for poor supervisor contact. Although empirical studies suggest that associations between dropout and student-to-student relationships are not as strong as with student-to-supervisor relationships (Bair & Haworth, 2004; Golde, 2005), we expect that exchange with other PhD students is negatively correlated with drop out from doctoral education (H7).
Having children potentially implies a restriction of time-resources that are available for other life areas such as doing research. Qualitative studies suggest that having children makes it considerably more difficult to balance home and academic demands, particularly for female PhDs (e.g., Lynch, 2008). Because a career in science requires a high degree of flexibility and the willingness to be geographically mobile, this career path might appear less attractive after the birth of a child. In addition, PhD positions may offer limited coverage of the higher cost of living typically associated with parenthood. Empirically, several US studies indicate that the number of children does not correlate with attrition (see Bair & Haworth, 2004). However, the timing of birth may be important. Mastekaasa (2005) found with Norwegian data that children born before the commencement of the PhD have no impact on attrition. Children born while enrolled have a clear negative effect on the probability of completion among female researchers. Negative parenthood effects were also found for Germany (Lörz & Mühleck, 2018). We therefore expect that doctoral students with children are more likely to drop out (H8).Footnote 1
Permanent employment offer
In Germany, individuals striving for a research career at a university usually have to deal with long periods of employment insecurity (Hüther & Krücken, 2018). Doctoral students almost always work in fixed-term, part-time positions. Against this background, getting a permanent employment outside academia might be an attractive opportunity for many doctoral students. As such a position might decrease time and motivation to work on the PhD thesis, we expect that doctoral students receiving a permanent employment offer outside of academia are more likely to drop out from doctoral education (H9).
Selection, mediation, and cumulative risk factors
To this point, we have derived hypotheses on direct effects of each theoretical component. However, our theoretical framework (Fig. 1) suggests that the theoretical components might be interrelated leading to selection effects and possible mediating mechanisms. For example, students from highly educated families are more likely to choose medicine than students from lower-educated families (Becker et al., 2010). Medicine, in turn, is expected to produce low dropout rates. We therefore expect that the choice of subject will at least partly explain the effects of parental education on dropout. Another example for possible mediation concerns scholarships. Because of the performance-based criteria for awarding scholarships, we expect that scholarships will at least partly mediate the effect of prior grades. Similarly, the degree of positivity of the supervisor-student relationship may be partly driven by a student’s ability and motivation. To address these issues, our multivariate models control for ability, field of study, and other observables (see subsequent section). While not the focus of this paper, we will discuss some of the mentioned interrelations below. However, we do not control for unobserved differences, and want to stress that our findings are descriptive, not causal.
Finally, some students are likely to be affected by several dropout risk factors at the same time, which makes them particularly vulnerable to non-completion. In the last part of the empirical analyses, we adopt a person-centered approach to illustrate the degree to which cumulative risk factors may increase the dropout propensity.