This is a very real thing, and it works both ways. The study I linked in another comment actually calls this out in the abstract: > Their scorings of pleasant

This is a very real thing, and it works both ways. The study I linked in another comment actually calls this out in the abstract: > Their scorings of pleasantness correlated negatively with the degree of MHC similarity between smeller and T-shirt-wearer in men and women who were not using the contraceptive pill (but not in Pill-users). I've twice had a very solid-to-that-point relationship just... stagnate and fall apart for no apparent reason after the woman involved went on birth control. The second time, I recognized the pattern and things went back to normal after she went off it (we're now married, with two quite healthy kids). On birth control, it seems the preference is for "similar genetic structures" in that region - which, when pregnant, makes sense. Hang out with family who will make sure you're safe. But if you meet/marry while on birth control, then go off to have kids, this stands a very real chance of causing problems. And they're deep enough that it's not something one can easily just will their way through.