Evolutionary Systems Therapy (EST) is an integrative psychotherapeutic approach grounded in the theory of evolution^[1]. It is one of the models explicitly developed from its inception as an evolutionarily informed psychotherapy. EST conceptualizes both mental health and psychopathology as inherently relationally evolved phenomena. In clinical practice, it combines various therapeutic techniques, drawing primarily upon mentalization-informed and compassion-focused strategies. Preliminary research indicates that this approach shows initial data regarding its efficacy in treating personality disorders, primarily schizotypal, schizoid, and obsessive-compulsive disorders^{[2][3][4][5][6]}. EST was originally developed by Simone Cheli and colleagues for the individual psychotherapy of schizotypal personality disorder^[7]. It has subsequently been progressively applied to different therapeutic formats and various other disorders (e.g., personality pathology broadly speaking, savant syndrome, etc.).

The theoretical background of EST integrates evolutionary biology with systemic epistemology^[8][9]. It conceptualizes human experiences as emergent qualities arising from continuous systems of self and other. The approach utilizes evolutionary theory to understand human variations across four primary domains: clinical, personal, temporal, and relational. Within this framework, psychopathology is not defined a priori; rather, it is viewed as a failure to adapt to a specific ecological and social context. Evolved psychological processes are considered non-optimal designs rather than perfect adaptations.

To understand individual traits, EST relies on the framework of Niko Tinbergen's four questions^[10]. This framework investigates how a psychological trait developed ontogenetically, the mechanisms through which it operates, its phylogenetic evolution, and its current evolutionary function. Furthermore, EST applies Life History Theory (LHT) to clinical psychology^[11][12]. LHT posits that organisms have limited resources to invest in development, leading to the adoption of "slow" strategies (characterized by later reproduction and higher parental investment) or "fast" strategies (characterized by earlier reproduction and lower individual investment) based on environmental pressures. EST suggests that specific psychopathological patterns, such as those found in borderline personality disorder or autism spectrum disorder, can be understood in relation to these fast and slow life strategies.

A core component of EST is the Social Brain Hypothesis^[13][14], which posits that the human brain, particularly the neocortex, evolved in response to increasing social complexity. Consequently, EST emphasizes mentalization^[15]—the capacity to understand one's own and others' mental states—as a distinctive and recent function of the social brain. Impairments in mentalization are viewed as central to specific human disorders, such as personality disorders and schizophrenia.

By integrating LHT with the Social Brain Hypothesis, EST identifies three interpersonal styles that correspond to evolved life history strategies and align with the three major super-spectra of psychopathology^[16]: the perfectionistic-affiliative, the antagonistic-distancing, and the schizotypal-defensive styles^[1].

• Perfectionistic-affiliative style: Corresponds to an affiliative strategy in which the other functions as an external validator of one's own value, predominantly associated with internalizing symptoms.
• Antagonistic-distancing style: Corresponds to a distancing strategy where the other is perceived as an obstacle to one's well-being or value, predominantly associated with externalizing symptoms.
• Schizotypal-defensive style: Corresponds to a defensive strategy where the other is viewed as a threat to well-being or survival, predominantly associated with schizotypal symptoms.

Ecological momentary assessment research on patients with personality pathology has suggested that these three interpersonal styles remain stable over time and are associated with the hypothesized corresponding spectra of psychopathology^[17]. Following treatment, these styles continue to characterize the individual's baseline organization, though they manifest with reduced intensity and are accompanied by significantly lower levels of associated psychopathology.

EST does not utilize a rigid, manualized sequence of techniques; rather, it employs a flexible framework tailored to the patient's developmental trajectory. The shared case conceptualization between patient and therapist is organized into three hierarchical levels. The macro-level focuses on the patient's developmental trajectory, distinguishing a basic personality organization from a maladaptive structure that emerges from persistent adaptation failures. The intermediate-level explores the development and function of prominent traits, utilizing a modified version of Niko Tinbergen's four questions regarding ontogeny, mechanism, phylogeny, and evolutionary function. Finally, the micro-level analyzes specific interpersonal situations and recurring interpersonal cycles, which are frequently driven and shaped by the patient's prominent interpersonal style. The therapeutic relationship is considered the primary tool of the intervention, serving as a safe social environment where patients can test and develop more adaptive relational strategies. The application of specific techniques—whether relational, autobiographical, psychoeducational, or experiential—is always managed collaboratively, with careful attention paid to how these interventions might intentionally or unintentionally impact the therapeutic alliance. To guide both individual sessions and the overall treatment plan, EST is organized into four dynamic priority modules:

1. Sharing: The therapist and patient collaboratively construct, revise, or update the clinical conceptualization of the patient's general functioning or specific evolutionary strategies.
2. Subjective Systems: Clinical work focuses on the intrasubjective dimensions of the shared conceptualization, primarily addressing self-to-self strategies and internal patterns.
3. Intersubjective Systems: The intervention targets the intersubjective dimensions, focusing on self-to-other relational strategies and recurrent interpersonal cycles.
4. Consolidation: The final module focuses on elaborating the meaning attributed to the therapeutic work, aiming to consolidate and generalize the newly acquired learning to the patient's daily social environment

Empirical evidence for EST is currently based on preliminary research, primarily focusing on individual psychotherapy for personality disorders^[2][3][4][5]. To date, the treatment protocol has been evaluated in a limited number of clinical studies, including one randomized controlled trial (RCT)^[18], one proof-of-concept single-arm trial^[17], and several case series, encompassing a small overall sample size.  Research has predominantly involved young adults diagnosed with various personality disorders, with a specific focus on Cluster A personality pathology. Preliminary findings report positive outcomes, including high remission rates and consistent reductions in both general psychopathology and specific indices of personality pathology. However, current literature notes that the limited number of studies and small sample sizes preclude broad generalizations about its overall clinical utility, necessitating further large-scale research to firmly establish its efficacy.

• Brüne, M., & Cheli, S. (Eds.). (2026). The Evolution of Psychotherapy: : Applying Evolutionary Theory to Mental Health Care. Springer Nature Switzerland. https://doi.org/10.1007/978-3-032-27009-2
• Cheli, S. (2026). Evolutionary Systems Therapy for Personality Pathology. In M. Brüne & S. Cheli (Eds.), The Evolution of Psychotherapy: Applying Evolutionary Theory to Mental Health Care. Springer (Preprint). [https://doi.org/10.31234/osf.io/rmp52_v1 Open access preprint].
• Cheli, S., & Lysaker, P. H. (Eds.). (2023). A dimensional approach to schizotypy: Conceptualization and treatment. Springer Nature Switzerland AG. https://doi.org/10.1007/978-3-031-41788-7 (Open access PDF).