The problem is that you're not considering all the other cases. Here are the possibilities with 2 siblings: Gender: B,B B,G G,B G,G # brothers:

The problem is that you're not considering all the other cases. Here are the possibilities with 2 siblings: Gender: B,B B,G G,B G,G # brothers: 1,1 0,1 1,0 0,0 Average # of brothers for boys: 0.5 Average # of brothers for girls: 0.5 You can see that in the families with a mix of boys and girls, the girls do indeed have more brothers. But this is balanced out by the families with more (or in this case all) of one or the other.