Tellegen and colleagues used data collected in the Minnesota Twin Study between 1970 and 1984 and the Minnesota Study of Twins Reared Apart between 1979 and 1986. They looked at well-being data for genetically identical twins and fraternal twins (~50% overlap in genetics). They looked at those who grew up together and a unique data set of twins separated shortly after birth. Identical twins who were raised apart (100% genetically identical, no shared environmental influences or experiences) turned out to be more similar with respect to their well-being than fraternal twins who grew up together (50% overlap on average and shared environment). So even though these identical twins had never met before the study, their happiness ratings were still more similar than the fraternal twins who grew up together in the environment. This indicates genetics clearly must play a role.
There are numerous references highlighted in the REF below. Based on these and metastudies of many thousands of cases (Bartels 2015, Nes & Roysamb 2015) it is estimated that approximately 30-40% of differences in happiness are accounted for by genetic differences between people while the remaining variance is accounted for by environmental influences and behaviors. It is worth noting that this is based on models that assume genetic and external factors are independent. Given our knowledge of epigenetics, this is not true: these dimensions interact and correlate.
“For example, some people have, due to the position of their eyes and the shape of their mouth, a more friendly-looking face than others. People in the environment unintentionally respond differently to people with more friendly faces. The shape of someone’s face is of course mainly driven by genetic background.”
REF World Happiness Report 2022 Ch.5 “Exploring the Biological Basis for Happiness”
3A1. Environmental factors play a key role in modulating gene expression (epigenetics).