To sane people, parents who kill their kids are both horrifying and tragic, though levels of acceptance and blame flow with cultural trends. Once upon a time, when a mother in Texas killed her children in a bath tub, celebrities like Katie Couric blamed everyone but the murderer. Today, there is a lot less exculpatory rationalization about killers.

A new paper in Forensic Science International invokes correlations to psychology and biology and therefore might be used to make filicide exculpatory once again - with enough data, epidemiology can prove anything. 

The paper is a statistical analysis of filicide in the United States, drawing on 32 years of data on over 94,000 arrests. The aim was to try and identify patterns and therefore warning signs. Along the way they debunk some myths. Women kill infants as often as men, for example, and step-children are not more likely to be killed, they are far less likely to be killed. The authors also delve into killings of adult children but that should really be a different paper. When singer Marvin Gaye was shot by his father at age 44, his father was obviously not biologically predisposed to it, nor had there been warning signs of any mental instability - Marvin had given his father the gun. But Marvin had also been verbally and physically abusive, something epidemiology statistics about parental killers don't highlight.

Statistical context

The data in the study came from the U.S. Federal Bureau of Investigation's Supplementary Homicide Reports (SHR) database. The authors of the paper looked at 632,017 arrests between 1976 and 2007, finding that 94,146 cases (14.9 percent) were filicides. The database includes information on the ages, genders, and races of the victims and alleged offenders, as well as the means employed to commit the murder.

"To know more about the epidemiology of this crime will hopefully help medical practitioners to identify people who are at risk for committing such crimes and that will help us with prevention, which is the ultimate goal of this research," said lead author Dr. Timothy Mariano, a third-year psychiatry resident at Brown University in their statement.

Prevention clearly doesn't mean banning guns. While gun ownership has gone up a lot, and has gone up even more with government efforts to ban firearms,the number of filicides has not only gone down since the early 1990s, it hasn't even matched population growth over the last three decades on a percentage basis.

72 percent of the children killed were age 6 or younger and around 33 percent were infants - children less than 1 year of age. Ages 7-18 found about 10 percent of children killed. Male children were more likely to be killed (58.3 percent) and only about 11 percent of victims were stepchildren.

Mothers were equally likely to kill an infant, but fathers were more likely to be the alleged murderer of children older than a year. Factoring out adult killings, fathers were slightly higher overall.

The most common filicide scenarios

A mother was slightly more likely to kill a daughter (19.7 percent of cases) than a father was (18.1 percent). The rarest instances were stepmothers killing either a stepson (0.5 percent) or a stepdaughter (0.3 percent).

Most common, including adult killings, was a father killing a son (29.5 percent of cases), followed by a mother killing a son (22.1 percent).

The most common method of killing was up close and personal - beating, choking, or drowning. That was the case in 69 percent of murders of infants. Parents rarely used blunt weapons, such as a baseball bat, or edged weapons, such as a knife.

A biological hypothesis?

They outline three hypotheses about potential biological causes. One is obviously that some parents who commit filicide have a mental illness. They mention low levels of the neurotransmitter serotonin, which is the focus some animal studies, and they note that the most typical ages of filicidal parents in the SHR data (18-30 years) also correlates to the age at which depression and schizophrenia are diagnosed and linked to serotonin.

Factoring out adult killings, which overwhelmingly involve men, makes the second hypothesis about sex hormones a little weaker. In animal studies, high levels of testosterone appear to coincide with higher rates of filicide.

The final hypothetical motive is also the weakest, especially given American culture where abortion is common, accepted, even late in the final trimester, and cheap. It invokes the evolutionary psychology motivation of "the unwanted child" and suggests that parents, particularly young mothers, may kill young children for whom they feel they cannot provide care.

Neither the statistics nor the hypotheses explain filicide, but they are at least a starting point.

"Hopefully future research will continue to improve society's ability to identify, manage, and treat populations at risk," they conclude.