The study looked at data gathered over 5 or 6 years from more than 12,000 identical twins, long considered the gold standard in environment vs genetics studies due to their identical genetic material. Using statistical analysis, the researchers found that set points among the 10 year old twins were very similar. But although their set points were stable (ie, they did not jump all over the map), they were not permanent, and as the twins grew older their set points became increasingly different, finally leveling out around age 60.
"Environmental experiences have a memory and stay with us. What governs the emotional set point of adults is a mixture of genetic factors and the total aggregate of environmental experiences," says author Kenneth Kendler. "If you want to be happy in old age, live a good life."