Imagine it is boom time for a group of humans. There's lots of food, and time for recreational and romantic activities. What behaviour will favour our genes?
There's no reason to worry about competition from strangers. And strangers are attractive. We plan to have 10 children and 100 grandchildren. If we mix in some slightly different genes then we might produce some offspring that combine good genes of ours with their good genes and make individuals with an advantage. So we're tolerant of strangers, and looking to have offspring with more than one partner.
Now imagine things are bad. Life is a struggle. The population is falling. What behaviour favours our genes now?
People that are like us share more of our genes. People who are different probably don't. And they're competing with us for the limited resources. Maybe we should cooperate with similar people and make sure we get our share relative to those who are different. And note that deaths from fighting are less of a loss to our DNA because deaths are common and it at least leaves more resources for other copies of our DNA. We now hope to have 2 children and 4 grandchildren, or maybe less. And it makes sense to marry our 2nd cousin, or even our 1st cousin.
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