The first gene that shows a correlation is DRD2, which also influences drinking behavior. Now, there are obviously some non-biological explanations for why that relationship might exist. After all, people who possess the gene are more likely to be social drinkers, which in turn means they're more likely to meet similar people at bars or parties. But the strength of the connection does seem to suggest there's a biological component to this as well, as people with the DRD2 gene consistently flocked together.