Antisocial behavior in children, especially when combined with 'callous unemotional traits' (a lack of guilt or remorse), can have a heritability estimate as high as 80%. This places its genetic influence on par with highly heritable disorders like schizophrenia and bipolar disorder.

The Hierarchical Taxonomy of Psychopathology (HITOP) model reveals that symptoms of mental health problems cluster into five major dimensions that closely correspond to the Big Five personality traits. This suggests mental illness can be understood as an extreme expression of normal personality variation.

A 50% heritability for alcoholism is linked to how one's brain responds to alcohol. Individuals genetically predisposed to feel more stimulated ('fun') from drinking are at higher risk, while those who feel sedated are more protected. The risk is about the positive reinforcement loop, not an innate tolerance.

Large-scale genetic studies suggest many distinct brain diseases (mania, depression, ADHD, Alzheimer's) are not separate conditions. Instead, they may be different expressions of a single, general genetic susceptibility to brain dysfunction, which researchers call "Factor P".

Counterintuitively, the heritability of traits like cognition and personality increases from childhood into adulthood. This occurs because individuals increasingly select and shape their own environments based on their genetic predispositions, a process that amplifies the influence of their genes over time.

The common 'hurt people hurt people' narrative is misleading for personality disorders. New research indicates a strong genetic contribution to traits like narcissism, which can manifest severely even in individuals who had no childhood adversity or trauma. Environment can exacerbate it, but the 'raw materials' are often innate.

It's a myth that all cells are a 50/50 blend of parental DNA. Neuroscientist Catherine Dulac's work shows that entire brain areas can be genetically identical to either the mother or the father. This explains why certain behaviors and traits are so strongly inherited from one parent.

Genes linked to addiction, impulsivity, and aggression are most active during fetal development, affecting the brain's fundamental balance of inhibition and excitation. This reframes addiction and conduct disorders as neurodevelopmental conditions akin to ADHD, rather than purely as choices or moral failings.

Citing research, Sam Harris finds it humbling how little control parents have over their children's character. He states that for most psychological traits, the breakdown is roughly 50% genetic and 50% environmental, but the environmental component is driven by peers and culture, not direct parenting.