Why is my sister related to me only 50%?

by TimFreeman 1 min read6th May 201116 comments


The standard story when dealing with inclusive fitness in evolutionary arguments is that my sibling's life is worth half of mine and my cousin's life is worth a quarter of mine.

But I obviously share more than half my genes with my sister because my parents are not unrelated.  My parents must share a lot of ancestors enough generations back that nobody has tracked them, since they resemble each other insofar as they both look human.  If I take into account that my parents are both human, I should be related to my sister much more than 50%.

So why do they assume a sibling has half your genes when reasoning about inclusive fitness?

My wife is Chinese, and both of my parents are of European descent.  Should I expect my kids to like each other less than I like my sister, because they are less closely related? 

This is an intellectual question about evolutionary psychology, not an anxious question about my family relationships.  We're all doing fine, don't worry.