Both genetic and environmental factors shape personality. Twin studies suggest that approximately 40–60% of personality trait variation is heritable (Power et al., 2015). However, these findings can be influenced by shared environmental conditions in twin rearing.

Power et al. (2015) also found that neuroticism and openness are more strongly inherited within families than traits like extraversion, agreeableness, or conscientiousness. This is notable for research on Borderline Personality Disorder (BPD) and eating disorders (EDs), as neuroticism—characterized by emotional instability, anxiety, and perfectionism—is a shared trait in both.