I've always liked the genetic explantion about Seasonal Affective Disorder (SAD). If your ancestors were "gathers" as in the "hunters and gathers" of food, then the hypothesis is once there's is less light the food is less and depression is more likely. This is "hardwired" into our make-up.

Considering that way of thinking, the "hunters" job was to get food. Even if things were bad in the cave, the need for food didn't stop. I imagine he would have to shut off his emotional circuits in order to fulfill the essential task at hand. The woman had to stay with the children and care for them because she was best suited for the task. The nurturing instincts and emotional feelings certainly were highly developed in such a role, as the males, this was survival.

It makes sense to me that this genetic hardwire/programing exists among the male and females today. Having it pop into my head from time to time helps me understand the Venus/Mars thing. JME/JMO