Most clinicians still underestimate the degree to which genetic factors shape personality traits and personality development. This is not controversial within behavioral genetics, but it remains surprisingly controversial in everyday clinical thinking.

Twin and adoption studies converge on a striking conclusion. Roughly half or more of the variance in most personality traits is attributable to genetic factors (Vukasović & Bratko, 2015). When combined with non-shared environmental influences, that figure exceeds 90%. What is notably absent is a large role for the shared environment, including broad features of upbringing that many psychotherapeutic models implicitly treat as decisive (Krueger et al., 2008).

This evidence has led me to revise my own thinking over the past few years. Like many clinicians trained in the psychoanalytic tradition, I once assumed that early relational experience held explanatory primacy for adult personality. I now believe that view substantially overstates what childhood environment can plausibly explain, even while underestimating what psychotherapy itself can still accomplish.

What the Data Actually Show

Genetic influence does not imply immutability or biological determinism. Traits can be shaped, moderated, and expressed differently over time, including through psychotherapy. What genetics challenges is not the possibility of change, but the assumption that adult personality structure is best explained by early childhood experience.

Shared family environment accounts for surprisingly little variance in adult personality. Siblings raised in the same home are often no more similar than strangers, once genetic relatedness is accounted for (Krueger et al., 2008). By contrast, non-shared environmental influences account for much of the remaining variance and often occur well outside early childhood (Plomin, 2011).

This creates a problem for therapeutic models that rely on a simple developmental narrative. In those models, enduring traits are often treated as sequelae of trauma, attachment failure, or parental misattunement. Such explanations are compelling, but they often exceed what the evidence can support.

Implications for Psychotherapy

If temperament and core personality traits are strongly influenced by genetics, then many of the features patients bring into treatment are not the products of childhood injury. They are part of the individual’s psychological constitution. Recognizing this does not diminish the role of psychotherapy; it simply clarifies its task.

Psychotherapy can and does help patients change. It can alter patterns of emotional regulation, interpersonal behavior, self-understanding, and the way personality traits are expressed over time. It does so not by locating a childhood origin for every enduring feature of personality, but by helping patients understand how their relational patterns repeatedly play out in the present.

Some contemporary psychodynamic approaches already reflect this reality. Treatments such as Kernberg’s transference-focused psychotherapy and Gunderson’s good psychiatric management explicitly acknowledge temperamental and genetic factors, and they focus less on excavating hidden childhood causes than on helping patients understand current patterns of affect, relationships, and behavior.

This distinction matters clinically. Affective intensity, impulsivity, suspiciousness, and rigidity may reflect enduring traits rather than the lasting effects of trauma. When clinicians assume otherwise, treatment can drift into elaborate reconstructions of childhood that are not well supported by the evidence.

Childhood experience still matters, but its role is often misunderstood. Development reflects an interaction between temperament and environment, with children shaping their environments as much as they are shaped by them. What looks like a pathogenic upbringing may in part reflect a difficult temperament eliciting more conflict or misattunement.

The arguments advanced here rest on a simple assumption that is not always made explicit: that what is true matters, and that clinical theory should be guided by the best available evidence rather than by tradition or preference.

Psychotherapy does not rewrite temperament, but it does not need to in order to be effective. Its power lies in helping patients live more flexibly and consciously with the psychological equipment they actually possess. A therapy grounded in that reality is not more limited. It is more honest, and ultimately more humane.