What do you mean? Do you mean the genetic explanation (as in "it was a mutation on the long arm of chromosome 20?") or do you mean the evolutionary pressures that were behind it?

I tried to explain the evolutionary pressures behind it - eg a person who couldn't signal would be unsuccessful, a person who signaled but never acted on those signals would be unconvincing. If you think those pressures wouldn't be sufficient, why not?

I don't have a play-by-play genetic explanation, but neither do a lot of well-established things.