(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.

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.

once every three days. Cheryl Meyer, co-author of several books on the subject, said it's probable that a mother kills a child somewhere in the US

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%).

Parents used what the study called "personal weapons" to beat, choke or drown victims in the majority of cases involving underage kids.

victims in the majority of cases involving kids. If the victims were adults, parents used guns in 72% of such killings.

There are several reasons why

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

Altruism: The parent kills the child because he or she may perceive it to be in the child's best interest. It may be reality-based (e.g., the child suffers from a terminal illness) or precede the suicide of the parent, as the parent feels it would be unfair to leave the child behind to face the cruel world.

Acute psychosis: The parent kills the child based on ideas that are inconsistent with reality. For example, the parent believes the child has been possessed by the devil.

Unwanted child: The parent kills the child that he or she regards as a hindrance.

Accidental: The child's death is an unintentional outcome of parental physical abuse.