The main purpose of the present study was to find evidence for possible differences in the prevalence and process of burnout between male and female GPs. By means of a three-wave panel study among a sample of Dutch GPs, we tested several theoretical burnout models and two possible sequences of burnout among male and female GPs separately.
Change in prevalence of burnout over time (research question 1)
Generally, we observed an improvement in the burnout status between 2002 (T1) and 2004 (T2), while between 2004 and 2006 (T3) the situation deteriorated again. These developments might partly be explained by several changes in the working life of GPs. Since approximately 2000, the out-of-hours primary care provision is more and more organized in large-scale GP cooperatives, with generally 40 to120 GPs taking care of populations ranging from 50,000 to 500,000 inhabitants . The GPs are assisted by doctor's assistants who perform telephone triage to prioritize treatment. Between 2002 and 2004 this development was at its peak. Research [8, 43] shows that this development has led to a decrease in the average number of hours a GP is on call during a week (from 19 to 4). Moreover GPs experienced a reduced workload and fewer problems in the separation of work and private life. We believe that this development could have had it's main effect between 2002 and 2004 and may explain the decrease in burnout between T1 and T2. The increase in burnout between 2004 and 2006 could be explained by the fading out of the positive effect of the GP cooperatives or by the implementation of a new Health Insurance Act ("Zorgverzekeringswet") on January 1, 2006, which already cast its shadow before 2006. This act had large (administrative and financial) consequences for GPs who had to reorganize their administrative systems and working methods.
We did not find significant sex differences in the change in burnout prevalence over time, although women seem to develop more emotional exhaustion over time, despite the fact that they worked fewer hours than the male GPs in our sample. In addition, men scored - on average - higher on depersonalization.
Process of burnout: causal order of the three dimensions of burnout among male and female GPs (research question 2)
As regards the developmental process of burnout, we found evidence for the fact that the aetiological process of burnout, that is the causal order of the three burnout dimensions, differs between male and female GPs. For men, depersonalization seemed to be the onset of burnout (note that men also had higher mean scores on depersonalization than women). Another salient finding among the male subgroup was that personal accomplishment seems to develop independently from the other two dimensions. Over time, we see that the feelings of personal accomplishment among male GPs keep increasing, despite feelings of depersonalization and emotional exhaustion. This finding suggests that for men reduced personal accomplishment is not a dimension of burnout. They appear to have a stable sense of self-efficacy that is not affected by exhaustion or cynicism.
For women, we found a strong influence of emotional exhaustion on depersonalization and of depersonalization on reduced personal accomplishment. In other words, for women burnout seems to be triggered by emotional exhaustion. We also found indications for a higher level of emotional exhaustion among women, particularly at time 3. This central role of emotional exhaustion for women is in line with, for instance, the Leiter and Maslach model. In addition, among women, feelings of personal accomplishment are affected by the other two burnout dimensions as well. Women who are exhausted and depersonalizing their patients, may start feeling guilty and less certain about their work and the quality of care they can provide.
These sex differences in the process of burnout are intriguing and may have large theoretical and practical implications. Below we describe several explanations for these sex differences in the developmental process of burnout.
First, the GPs' circumstances may have led to several gendered processes. The abovementioned introduction of the Health Insurance Act, for instance, along with all the hectic around it, may among men have lead to attracting management tasks and delegating caring and patient related tasks, while women may have tried to hold on to taking care of the patient at any cost; they did not depersonalize, but became exhausted. This feminine tendency to interact and communicate with patients on a high quality level may have been hampered, and this in turn led to exhaustion.
Second, women may have or perceive other (working) conditions and other individual characteristics than men, which in turn leads to a different development of burnout, with emotional exhaustion being more salient for women. Several studies [44, 45] have indicated for instance, that women report more negative interaction between work and family life and that this interaction affected the development of emotional exhaustion only among women . The same study  indicated that the individual factor "goal orientation", which was slightly more prevalent among male physicians, has a preventive effect for emotional exhaustion for men, but not for women. Goal orientation was defined as the active orientation towards long-term goals coupled with a reliance on one's own actions and feelings of personal responsibility to obtain these goals.
Finally, as mentioned above, men have a tendency to select avoidance coping strategies . The construct depersonalization may also be considered an avoiding coping strategy, which may easily be adopted by men as a method to deal with a stressful situation. Our findings indicated that for men depersonalization leads to emotional exhaustion. This underlines the idea that psychological withdrawal can be an ineffective and dysfunctional coping strategy, which may affect the well-being, behaviour and performance of an employee. Moreover, for overly detached - male - GPs it might be more difficult to create a relationship with a patient, which may in the end influence the provision of care. This "indifferent" behaviour may cause patients to become more and more demanding, and at the same time cause feelings of overload within the GPs, which in turn triggers emotional exhaustion, and then the burnout process has started. At the same time, however, it should be noted that a certain level of detachment (i.e., detached concern) can be a very useful strategy for the prevention of ill-health and malfunctioning. In this light, one might argue that for both male and female GPs in our sample, a certain level of depersonalization might be a protective factor against burnout.
Several methodological reflections should be made with regard to the present study. First, in the present study we aimed to find evidence for causality of relationships. However, in observational research, even if it is longitudinal and includes three waves, causality cannot be proven. Randomized controlled experiments, which are impossible and unethical to conduct in this research area, are needed for this purpose. In field studies, only causal inferences can be drawn .
In the second place, we measured the study variables at three fixed time points, with two year time lags while the processes we observed are continuous. Hence, there may have been a misfit between the actual causal lags among the burnout dimensions and the time lags used in the study. Moreover, the true time lags for the three burnout dimensions may differ from each other as well. In a study that uses a time lag of two years, a true three-months time-lag, might be better represented by the synchronous effects than by the lagged effects . This time-lag problem is very difficult to solve and is intrinsic to longitudinal research in general. As theories rarely specify the correct causal lags, the causal processes may be captured best using different time lags in a study. Nevertheless, a misfit between the time lag used in the study and the actual causal lag, results in an underestimation of the true causal effects .
Third, there is the problem of attrition (i.e., panel mortality) that hampers CLPD. We used listwise deletion to arrive at the panel groups (complete data) for our cross-lagged panel analysis, and the panel groups in our study consisted of about 30% of the initial sample. In addition, our non-response analyses showed that the dropout among men was slightly higher than among women. This may have biased the results because of selective responses. The panel group did not differ from the dropouts with regard to the levels of burnout, though. Therefore, we concluded that no serious selection problems occurred. This type of missing data is called stratified MCAR and this could justify the use of listwise deletion as a method to handle these missing data when sex is taken into account in the analyses, as is the case in our study . An issue related to the latter is the fact that in our study sample female GPs were overrepresented in comparison with the total population. Because we analyzed men and women separately we did not apply survey weighting. We made the decision to use a disproportionate sample because we aimed to establish sufficient variation in exposure to the model variables among both men and women. Moreover, female GP trainees account for 70% of the total group of GP trainees, so this study may also shed some light on future cohorts.
Finally, it should be noted that in the present study we ignored contextual and personal influences on the development of burnout. Hence, we did not control for the possible confounding effects of, for instance, work and personal characteristics, and gender characteristics (i.e., masculinity and femininity).
Further research and practical implications
In spite of the reflections discussed above, we believe that the results of this study have important theoretical and practical implications, and give some directions for future research. First of all, the results implicate that in the current GP population there is no one valid burnout model for both men and women. This finding has implications for both the theorizing in the field on burnout, but also for preventive interventions with regard to burnout. These findings should be replicated in more longitudinal studies in other occupational groups. Moreover, the results of this study imply that in future burnout research it is advisable to consider both sex and gender differences, and maybe cultural differences as well. The present study particularly focused at sex differences between men and women. In future research though, it might be interesting to include the concept of gender identity (i.e., masculinity and femininity) . Gender characteristics can vary considerably between individuals of the same sex, and therefore, sex and gender differences ought to be taken into account in future research regarding burnout . It is interesting to focus on gender dynamics among physicians because the proportion of female physicians is increasing so rapidly and because we learned from previous research that professional identity and gender identity are profoundly influenced by the professional socialization in a hierarchical masculine culture in medical school and medical work.
With regard to practical implications, the results of this study suggest that in the first place it seems wise to pay attention to the mental health of general practitioners, and to take gender differences into account when doing so. For men, it might be wise to focus at coping capacities, in order to avoid withdrawal and isolation. For women (for whom emotional exhaustion is the onset of the burnout syndrome) it might be more useful to focus at the specific determinants of this fatigue-related dimension, and on learning how to prevent exhaustion by boundary-setting. In addition, Maslach and Leiter  suggest to detect burnout at an early stage and prevent the full blown development of burnout. They suggest that people with either high levels of emotional exhaustion or depersonalization show an "early warning" pattern. This would mean that for instance in screening for early burnout, we should focus on emotional exhaustion for women, and on depersonalization for men. This study also shows a need for gender-tailored stress interventions, which is in line with recent findings  about men and women reacting differently to stress-reducing interventions, and with recommendations from the European Commission  which has recognized a still-existing, large gender gap in almost all aspects of work quality, reconciling private and professional life and health and safety at the workplace. In this light the EC recommends the implementation of innovative and flexible work and leave arrangements.