Increasing Heritability of BMI and Stronger Associations With the FTO Gene Over

Twin research has been pivotal in identifying the substan-tial genetic influence on individual differences in BMI (1–5). However, the genetic effect size has varied markedly between studies, perhaps in part because of age differences. For a broad range of traits such as externalizing behavior problems, social attitudes, intelligence quotient, anxiety, and depression, herit-ability increases from childhood to early adulthood (6). There is also evidence for developmental increases in heritability for exercise (7), sports participation (8), and eating attitudes and behaviors (9). However, the evidence is mixed for age changes in the heritability of BMI. Birthweight is ~40% heritable (10); moderate heritability has been reported at age 4 (ref. 11) and high heritability at age 10 (ref. 5). In adolescence, herit-ability estimates have been twice as high (ranging from 70 to 90%) (4), but the heritability of BMI in adulthood is generally lower, and there is evidence that it decreases over the life span (e.g., ref. 12); a recent study found decreases in heritability from 12 to 16 years (13). In contrast, research into BMI in a sample of 1,412 twin pairs showed that in three age groups (8–10, 11–13, and 14–16 years), heritability increased from 0.67 to 0.88 to 0.93, respectively (3).Changes in heritability over the life span could be due to genetic or environmental changes. It is possible that different genes influence a trait at different developmental stages or the same genes could have a larger impact on a trait as it develops. In the case of obesity, heritability could change because individuals at genetic risk for obesity increasingly select obesogenic environments correlated with their genetic propen-sities. Twin studies can evaluate developmental continuity for genetic and environmental influences using multivariate longi-tudinal genetic analysis, which considers not only the variance of traits considered one at a time but also the covariance among traits. It yields a statistic called the