Before the human apes got smart, they were social animals who lived in groups like baboons or chimps or gorillas or dogs or chickens. And like all those other animals, we have embedded in us a really strong pecking order system to prevent rivalries from causing too much damage to group survival. Basically the dominant male or female just has to glare or flair their nostrells or snort loudly and that is enough for a lesser membe to back off. We have been social animals for maybe 50 million years but we only began rational thinking about 50 thousand years ago (or whenever we began painting). So here is the deal, something that has been part of us for 50 MILLION years! is not going to go away in 50 thousand. So what happens when rational thought comes up against this powerful embedded part of the human animal? I believe that the rational thought often gets shut off.
The theory is that our thinking brain gets turned off automatically "without us even being aware that it got turned off!" and this mostly happens when rational thought comes into conflict with stuff like pecking order, or sexual opertunities.
Nowadays, pecking order is harder to define because you can be in several pecking orders at once in different positions. For instance, the doctor might be high up on the medical pecking order, but if he is married to your sister, he may be very low down on the "family" pecking order. What do we say very soon after meeting someone? "What do you do", is pretty high up there and all you do with that question is establish a pecking order very quickly. After that you can close down your rational brain because you know your place whether it is high or low.
Now, I know this is not a nice theory and I am just a stone mason
but before you write it off, check this very recent link about female competition. (I don't have a male link handy, but Bill Clinton will do for the moment as a good example)
Tracy Vaillancourt did an experiment http://www.ottawacitizen.com/entertainment/Meeeoooow+claws+come+when+wom...
We all assume that the women in the study are obeying "social" rules. But suppose it is something much deeper and millions of years older?