The present single centre study investigated the preferences among medical students concerning the selection of a postgraduate specialty and their reasons behind this process.
It should be noted that, according to our study, the interest in surgery does not decline during the course of studies, as reported previously by Ansorg et al.  and Paulmann et al. , but remains constant. Ansorg et al. and Paulmann et al. observed a declining interest in surgical fields, or rather a lower proportion of female students interested in surgical fields. In contrast, Diderichsen et al.  reported an unchanged interest, which is in accordance with our results. This is further supported by the national statistics regarding the distribution of board-certified physicians [29,30,31]. According to the German Federal Statistical Office, the rate of surgeons remained stable at 20% over the last 16 years [31,32,33].
Specialty preferences and comparison with working physicians
According to our results, the distribution of preferred postgraduate specialties among Jena medical students is comparable to the current distribution of specialties among currently working physicians in Germany [Fig. 1]. [29,30,31,32,33]. Selected specialties of the study population were compared to statistics for German physicians . The distribution of medical specialties among board-certified working physicians and the preferences of the students was very similar, suggesting an unchanged interest in the different fields of medicine [see also Table 5] [29,30,31,32,33]. This corresponds to the findings of Diderichsen et al. , who stated, in their cross-sectional study, that Swedish medical students show nearly the same preferences in their specialty selection, irrespective of their gender. The proportion of students selecting surgery in Jena increased slightly from 21.4% in 2014 to 27.6% in 2016. If our results hold true, the potential shortage of surgeons would be limited to what is required in order to meet the growing demand from a population with increasing age and healthcare needs. As female students outnumbered male students, the shortage in surgical residents due to family-related absences requires a rethink regarding working time models in understaffed medical fields [4, 7].
The group of undecided students is of particular interest, because of the possible recruitment of these students in medical specialties with a shortage of residents. According to Al-Fouzan et al. , the proportion of undecided students could be diminished with the help of formal career counselling during medical school. This could be implemented according to the Canadian guidelines . An optimal time to perform career counselling is the end of the clinical part of studies, as well as the beginning of the residency, which could be carried out as individual counselling or as a group event, especially for disadvantaged students . In addition, there may be a lack of role models in some understaffed specialties [5, 36]. Role models are characterized by their expertise and, in particular, their approachability .
In our study, the reasons rated as important by the majority of undecided students were different to the reasons given by students primarily interested in surgery. Consequently, the factors “workload” and “reconciliation of work and family life” should be taken into account when trying to attract currently undecided students into potentially under-represented specialties, such as surgery. In other words, special effort should be focused on offering suitable working conditions to this cohort of students.
Factors influencing specialty selection
We identified the factors influencing the selection of postgraduate medical specialties including personal motivations, career-associated reasons and work-life balance. Personal aspects of future life planning and characteristics attributed to a given specialty had a large influence on the selection of the postgraduate specialty.
Characteristics commonly attributed to surgery include promising career prospects, high workload and good reputation. Kiolbassa et al.  also stated that students selecting surgery are more concerned about their career prospects and their reputation than students selecting other disciplines. These characteristics were also the key influencing factors among students selecting surgery as their postgraduate specialty in our study, as well as in the survey conducted by Khader et al. and other studies [18, 19, 28, 37,38,39,40,41]. According to Khader et al. , in the main, male students preferring surgery were highly influenced by factors such as prestige and income, whereas females did not rate these factors as being of the utmost importance.
In contrast, Kaderli et al.  and others [5, 19, 43] stated that factors such as work-life balance and family planning do not equate with seeking a undemanding lifestyle, but with having the time to fulfil life goals besides work . Consequently, surgery is less often selected by students who attach importance to family life, as a complement to their working life. These students tend to select internal medicine, diagnostics or psychiatry as their preference. Similar results have also been reported in studies by Alers et al., Takeda et al. and others [18, 19, 44,45,46,47]. According to Alers et al. , Diderichsen et al.  and Harries et al. , female physicians tend to work part-time more than their male colleagues. These women often select disciplines such as general medicine or internal medicine because they associate them with family friendliness [28, 45]. Takeda et al.  and Correia Lima de Souza et al.  stated that surgical specialties (surgery, neurosurgery) were associated with having little time for the fulfilment of life goals, whereas other specialties such as ophthalmology, radiology or dermatology allowed time for personal goals. In particular, more female students would prefer to work part-time compared with male students [28, 48]. Consequently, female students follow a different reasoning process in the selection of a specific specialty than male students. Furthermore, the feminisation of medicine, and in particular of surgery, would require a change in thinking about work-life balance, modern working time models, and the participation of females in leadership and research .
In summary, the preferences of the medical students in our study almost reflect the distribution of future physicians across Germany [Table 2] . Therefore, the influencing factors for the decision-making process should be taken into greater consideration when addressing a shortage of physicians in certain specialties [50, 51]. For instance, improved working conditions would be required to accommodate the wish for a sustained work-life balance [27, 31]. This would be essential not only to attract students into under-represented medical fields at the postgraduate specialty stage, but also to help them complete their residency and enable them to pursue a successful career as a physician. However, our results confirm that there is a need for the introduction of more flexible working models, career counselling at an appropriate stage during studies, and good mentoring interventions for residents.
This study was conducted at a single medical school in Germany, but as a repeated cross-sectional study addressing all students repeatedly over 3 years. This design has both strengths and limitations. Including other medical schools could have provided a better representation of the study population. However, our sample population had a similar gender and age distribution as nationally reported, suggesting a satisfactory representation.