Actually, I think it's the gathering instinct. Men are typically considered the hunters. They like to go out, seek out a target, and make the kill. The women are the gatherers. They are programmed to search for berries, roots, and other treasures and to bring them back to the den. That is why men get impatient with women and go hang out in the tool section of the store... stalking their next purchase. Women, meanwhile, go about their business... looking through racks of clothes and searching the sales aisles while they collect bags of treasures to bring back to the den.
This Wikipedia link may help, but doesn't much talk about the theory that women were the gatherers while men were the hunters. http://en.wikipedia.org/wiki/Hunter_gath… Personally, I think the reason women were the gatherers is that they needed to regard the safety of the children that were likely in their immediate care.
