I would say that that’s kinda the idea of predispositions. It means that it is one factor among many. In many cases studies have found that the similarities attributed to genetic similarities account for only a reasonably small variance. On top of that, you’re assuming that that’s “need to spread offspring” is the only factor in play, when it pretty much isn’t. Just as you might assume there are behavioural or societal reasons for this (and i’d probably agree, evolutionary psychology being about predispositions), it’s fair to say that there could well be factors that hinge on other genetic predispositions besides that one.
You seem to be assuming that because such predispositions may not come to fruition, that genetic factors are insignificant (I assume you’re still holding to your hooey idea?). A change of 40-50% still leaves a significant percentage.
It’s pretty hard to do such tests, as not every psychologist has access to or the funds for a genetic test of the type you’re talking about. But similar tests can be and are done with those naturally provided genetic testers - twins. Twin studies are a pretty big linchpin in evolutionary psychology (among other things), and significant results are got when looking at certain traits by comparing fraternal and identical twins. The problem, as has been pointed out, is that often we can’t then look through and pick out the exact genes, or combination of them. But we can know they’re in there somewhere.
Certainly it would be back to the drawing board if the solution was found lacking; but as of yet, it really hasn’t been. There’s good evidence for genetic predispositions towards certain behavioural traits.
Taking into account that it was your apparent understanding of evolutionary psychology that psychologists are claiming that genes will with 100% certainty determine our behaviour, I would suggest to you that such a misunderstanding of the basic theories tends to suggest you yourself should learn the lesson and go back to reexamine the evidence. Seriously, pick up a good book on twin studies.