Whereas it is evident that psychotherapy works, it is still not exactly clear how it works nor precisely what works in psychotherapy. There is considerable disagreement about the factors responsible for eliciting psychotherapeutic change. This refers to the mixed results of comparative psychotherapy research: On one hand, comparisons of different psychotherapy approaches revealed only minor effect-size differences. Meta-analyses showed an average effect-size difference of about 0.2 between different kinds of psychotherapy. This finding has been labelled the “Dodo bird verdict” or "equivalence paradox". On the other hand, however, it has been repeatedly shown that some psychotherapy approaches are superior to others in the treatment of certain mental disorders and in the treatment of patients with certain interactional characteristics. These inconsistent findings of comparative psychotherapy research has led to two rivaling assumptions about the therapeutically active factors in psychotherapy: the specific ingredients assumption and the common factors model.

In the "Taxonomy Project" we are trying to resolve the inconsistencies. Survey data among experts and empirical observations in psychotherapy clinics have shown that specific ingredients ('therapy techniques') and common factors are intrinsically related rather than mutually exclusive. Ultimately, our goal is to describe which therapy techniques realize a certain common factor.

In recent decades, embodiment has become a frequently cited construct in psychology and cognitive sciences. Researchers and practitioners use this construct to denote the  position that mental processes (cognition, thinking, emotion, the psychological self) should be viewed in the context of the moving body. By this they depart from the 'computer metaphor' of mind – the embodiment stance instead posits that abstract information processing is not the essence of cognition. Accordingly, the mind cannot be fully understood without considering its embedding, the body. This has far-reaching implications for psychological research as well as for practical applications such as psychotherapy. The conventional view (including folk psychology) would emphasize that environmental stimuli entail mental (cognitive and perceptual) responses in a perceiver, which may result in emotions and bodily behavior. Embodiment complements this view by acknowledging the less evident, but equally important, reverse sequence – motor action and body postures may have an impact on the mind, often at an unattended and tacit level. Both sequences together comprise the bi-directionality of embodiment, and it is especially the James-Langean body®mind-processes that are the focus of embodiment research.

What is true for the cognition of individuals has implications for social interactions between individuals. It was consequently found that embodiment shapes social cognition and the way people communicate as well. When we observe another person, we automatically employ capacities of perspective taking, sometimes termed Theory of Mind (ToM) or mentalizing, that enable us to perceive the world almost "through the other's eyes". This may be studied empirically as social contagion, mimicry, or synchrony. Especially emotional utterances can become socially "contagious" und may thus entail synchronized behavior. In a series of projects we showed that psychotherapy dyads were significantly synchronized in their nonverbal behavior during sessions. Synchrony was also meaningfully correlated with patients' interactional problems, self-efficacy, and attachment styles (Ramseyer & Tschacher, 2011). Therapeutic dyads with higher synchronies had increased patients' self-efficacy as an outcome. Insecure attachment patterns of patients, distress due to interpersonal problems and higher levels of psychopathology were all correlated with lower synchrony during sessions.