monozygotic twins = big effect
An overview of recent books on happiness, in the New York Review of Books.
...Beginning in the 1980s, Lykken and his colleagues surveyed 2,310 pairs of identical and fraternal twins, some reared together, others brought up apart, looking to see how closely mood, affect, temperament, and other traits tracked with shared genes and/or a shared environment.
What they found (from a smaller subset of the original group) was that the "reported well-being of one's identical twin, either now or 10 years earlier, is a far better predictor of one's self-rated happiness than one's own educational achievement, income, or status." This held not only for identical twins raised together but for those brought up apart, while for fraternal twins raised in the same household, the likelihood that one's sense of well-being matched one's twin's was, statistically speaking, not much greater than chance.
Original research by the Lykken group.
Happiness Is a Stochastic Phenomenon
David Lykken and Auke Tellegen
University of Minnesota
Psychological Science Vol.7, No. 3, May 1996
Happiness or subjective wellbeing was measured on a birth-record based sample of several thousand middle-aged twins using the Well Being (WB) scale of the Multidimensional Personality Questionnaire (MPQ). Neither socioeconomic status (SES), educational attainment, family income, marital status, nor an indicant of religious commitment could account for more than about 3% of the variance in WB. From 44% to 53% of the variance in WB, however, is associated with genetic variation. Based on the retest of smaller samples of twins after intervals of 4.5 and 10 years, we estimate that the heritability of the stable component of subjective wellbeing approaches 80%.