A new study in the journal of Forensic Science International looked at three decades worth of filicide (the killing of one's son or daughter) cases between 1976 and 2007 and found they occurred about 500 times a year in the US.

The study also found out that almost 72% of those killed by their own parents were 6 years old or younger. And one-third of the victims were just babies under 1 year of age.

Also, 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.

However, only 10% of the victims were killed by their stepparents, which means that 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%). Parents used what the study called "personal weapons" to beat, choke or drown victims in the majority of cases involving underage kids. If the victims were adults, parents used guns in 72% of such killings.

The study also went on to analyse why these killings happen and 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.