Hoarding in females is partly genetic, not just learned behavior.

Shared family environment plays surprisingly little role.

Male heritability remains an unanswered question.

Where does hoarding behavior come from? Is it something people choose, something they learn, or something encoded in their DNA? A landmark 2009 study published in The American Journal of Psychiatry by Alessandra Iervolino and colleagues tackled this question, and what they found is both surprising and fascinating.

The setup: Twins as nature’s experiment

The researchers turned to one of behavioral science’s most elegant research tools: twins. Identical (monozygotic) twins share virtually 100 percent of their DNA, while fraternal (dizygotic) twins share on average about 50 percent, roughly the same as any two siblings. If a trait like hoarding is heavily influenced by genes, identical twins should resemble each other far more than fraternal twins. If not, then shared family environment and experiences are probably doing most of the heavy lifting.

The study drew on a large sample of 5,022 identical and fraternal twins who completed a validated self‑report measure of compulsive hoarding, drawn from the TwinsUK registry. With this sample size, the researchers had sufficient statistical power to trust their estimates. They used liability‑threshold models and maximum‑likelihood structural equation modeling to tease apart how much of the variance in hoarding could be attributed to additive genetic factors, shared environmental and nonshared environmental factors, conducting the model‑fitting analyses in the female twins, who numbered 4,355.

A total of 2.3 percent of twins crossed an empirically derived threshold for clinically significant hoarding symptoms on the questionnaire, with notably higher rates in males (4.1 percent; a smaller sample) than in females (2.1 percent). That gender gap is intriguing and still not fully explained, though the authors flag it as worthy of future investigation.

The headline finding, though, was heritability. In female twins, genetic factors accounted for approximately 50 percent of the variance in compulsive hoarding, with nonshared environmental factors and measurement error accounting for the other half. Put differently,........

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