(CNN)—It looks like it has happened again. A young male has allegedly killed. In this case it is nine people at a church in South Carolina. Anyone who is paying attention hears the news and thinks, "Here we go again." But why? After spending 20 years interviewing killers as a psychological expert witness in murder cases I have some thoughts about this. Males are disproportionately over-represented in the ranks of America's killers -- about 90%, in fact. Why is that? Some of it appears to lie in the biological vulnerability of males. About 30% of males (versus 9% of females)...