Chronic fatigue syndrome (CFS) has always been surrounded by controversy. Whether pertaining to its etiology or treatment, it appears as if there is no agreement on any aspect of CFS [1]. However, there is one fact virtually everyone acknowledges: individuals with CFS are seriously fatigued and severely limited both physically and psychosocially. This self-experience is real and undeniable, and in this regard, persons with CFS differ from other individuals. Consequently, ever since Millon et al. [2] conducted the first small-scale investigation on the subject, the study of personality and CFS has been an expanding field of research. While the evidence in this area has generally been somewhat inconclusive, higher emotional instability and to a lesser extent introversion have consistently been linked to CFS. Now, Poeschla and colleagues [3] confirm these most robust findings in a well-designed, large-scale, co-twin control study. Beyond this, they further the insight in these relations through an examination of the underlying genetic mechanisms.

Since its inception, the study of persons with CFS has been characterized by two complimentary approaches to personality [4]. The first approach to personality and CFS focuses on general dispositional traits, which are considered to be stable across time and context [5]. These are usually studied through standardized questionnaires and structured interviews. The main benefit of this approach is that it produces objective, quantifiable data that enable the comparative study of patients and controls. The downside to this approach is that it is not directly clear how findings on the trait level relate to individuals' real-life experiences and behavior. As a consequence, and because traits to a large extent are regarded as unchanging, it could be argued that their only clinical importance is as factors influencing a treatment's potential success [6].

The second approach to personality and CFS focuses on personal identity, which is considered to be continually evolving and dependent on context [7]. This is usually studied through an analysis of patients' self-narratives. The main benefit of this approach is that it can remain nearer to the subjectively experienced reality of the person with CFS and might therefore serve as a therapeutic target. Just as with personality considered on a trait level however, the advantage to this approach also entails its main drawback. Because of their qualitative nature, the obtained data are contingent and often difficult to compare with controls in a proper methodical way. Therefore, the main challenge for this approach is to conduct longitudinal (intervention) studies and to do so in a quantifiable and systematic manner [8].

For the trait approach to personality and CFS, two main directions of research seem vital. In the first place, longitudinal studies—like the Kato et al. [9] study that the authors refer to—are of the utmost importance. Not only in order to further investigate whether personality traits can predict CFS-like conditions, but also to investigate whether therapeutic interventions can target them. As the authors indicate, by learning to cope with their own traits, such strategies might increase patients' well-being and health. Interestingly though, they also seem to suggest that certain personality traits might change (back) as a consequence of clinical intervention. Related to this point, in the second place, more studies into the causal pathways and genetic mechanisms underlying the relation of personality and chronic fatigue are needed. With intriguing results, such as the finding that emotional instability is associated with chronic fatigue in the dizygotic twins, but not in the monozygotic twins, and a thorough discussion of the clinical implications of their research, the present article provides an excellent example of such a study.