Kevin Mitchell wrote this about how to think of genetic influence, which I think is very important. As he explains, on the population level (when you take the means) there seems that genes have a linear additive effect. As his example he uses height, which is coded at many many different loci, and where there seems to be evidence that the more of these that have the ”tall” variant, the taller that population will be. On average. But, it really doesn’t say, the taller the INDIVIDUAL will be, as he explains. Over populations, variation and messiness may disappear.
And, then he goes on explaining how interactions still can happen, by using highly inbred mice as his test population.
I think this is something to really remember in psychology. We test on populations, but frequently our interest (at least as people) are not about populations, but individuals.