This is from a recent paper by Angela Coolsa and Eleonora Patacchini:
We examine the impact of having a younger brother on females’ adolescent environment and adult earnings….Girls with a younger brother earn about 7 percent less in their adulthood.
I don’t know what to say about this. I’m sorry, Karen. You’d be making 7 percent more if it weren’t for me. That’s thousands of dollars I’ve cost you! Each and every year!