(CNN) It's the most unfathomable of crimes: a parent killing their own child. It grabs headlines when it happens -- like this week in Georgia, where a mom is accused of stabbing four of her children to death. The reality is, filicides happen with depressing regularity in the US. And mothers are almost as likely to be the killers as fathers.
Of course we're shocked whenever we hear about a parent taking the life of their own children. Think Andrea Yates or Susan Smith. But what's even more shocking (and sad) is that this type of crime happens a lot. A study in the journal Forensic Science International looked at three decades worth of filicide cases (between 1976 and 2007) and found they occurred about 500 times a year in the US.
But more than 13% of the victims were adults, specifically people in ages running from 18 to 40 years old. So the threat of filicide doesn't go away when people get old enough to move out on their own.
More than 40% of the killers in these crimes were mothers, with fathers making up about 57% of those who killed their own offspring.
Cheryl Meyer, co-author of several books on the subject, said it's probable that a mother kills a child somewhere in the US once every three days.
Only 10% of the victims were killed by their stepparents. That means 90% of the victims were the biological sons or daughters of the killer. And sons (52%) were more likely to be killed than daughters (38%).
Dr. Timothy Mariano, the study's lead author, offered up three theories: the parents are often mentally ill, they usually have higher levels of testosterone and the offspring that they kill may be considered unwanted.
Forensic psychiatrist Phillip J. Resnick, pioneer in the study of filicide research, identified five major reasons: