Hugh Pickens writes "Studies comparing identical twins with non-identical twins have helped to establish the heritability of many aspects of behavior. Recent work suggests that about one third of the variation in people's happiness is heritable. Jan-Emmanuel De Neve has taken the study a step further, picking a popular suspect — the gene that encodes the serotonin-transporter protein, a molecule that shuffles a brain messenger called serotonin through cell membranes — and examined how variants of the 5-HTT gene affect levels of happiness. The serotonin-transporter gene comes in two functional variants—long and short and people have two versions (known as alleles) of each gene, one from each parent. After examining genetic data from more than 2,500 participants in the National Longitudinal Study of Adolescent Health, De Neve found that people with one long allele were 8% more likely than those with none to describe themselves as very satisfied with life and those with two long alleles were 17% more likely of describing themselves as very satisfied. Interestingly enough, there is a notable variation across races with Asian Americans in the sample having on average 0.69 long genes, white Americans with 1.12, and black Americans with 1.47. 'It has long been suspected that this gene plays a role in mental health but this is the first study to show that it is instrumental in shaping our individual happiness levels (PDF),' writes De Neve. 'This finding helps to explain why we each have a unique baseline level of happiness and why some people tend to be naturally happier than others, and that's in no small part due to our individual genetic make-up.'"