Nice hypothesis and a clever first stab. You should definitely develop the idea further...up to the standard of publishing. Consider PLoS One.

Which cultures are dad and cad cultures?
I've never heard of . What about populations from the Near East or South Asia? How do they compare in their frequencies of the 7R allele? Is it the "caucasoid cluster", and why would East Asians be more of a dad culture than the West Europeans? Are they any less polygymous of a region, historically?

Your hypothesis does not make sense. Recent selection would only result in the manner you mention only if west africa suddenly switched from a dad society to a cad society, causing those with the genes to suddenly have more success at reproduction.

A more likely theory is that life in Nigeria suddenly became hard and the people who survived were those who were less anxious and more optimistic about the future. The dad/cad split is likely a result of the change in the population rather than it's cause.

Remember, the dopamine system is involved in several things like increased happiness, motor skills, mood, creativity, learning by experience, creativity,sleep. Lower dopamine on the other hand has been implicated in social anxiety, depression, parkinson's disease, decreased motor skills etc.

It would seem that a society that's more stressful and difficult to navigate would require people to be high on dopamine. If you get all depressed cos your wife died during childbirth or your child died of malaria, you'll never get over it in time to get another wife and have more children.

I think generally, African's have been shown to be better off in the mood department. The long form of the serotonin regulator 5HTT has also been shown to be more common in africans. This form has been associated with the ability to recover from life's stresses without resorting to depression or suicide.