In 1677, an unmarried New York woman named Elizabeth Rainer ruined herself. According to an indictment filed against her, she had "played the whore & become with childe by fornication." Rainer had tried to hide her pregnancy. After giving birth furtively, she took her baby to a nearby workshop where she "didst sinfully & wickedly leave it dead upon a piell of Chipps."

The death penalty was widely applied in the early colonies. In fact, it was the normal punishment not only for murderers, but also burglars, arsonists and counterfeiters. But the court spared Rainer's life. She was merely required to stand 30 minutes on the gallows with a halter around her neck -- the 17th-century equivalent of a short prison term. In that primitive age, maternal infanticide was often seen as something less than true murder.

But that age never really ended, at least not in Canada. Our criminal code deals gently with mothers (not fathers) who kill newborns before having "recovered from the effect of giving birth." One mother who may come under this provision is the 15-year old Brampton, Ont., girl accused of stabbing her baby to death immediately after delivering it in her parent's bathroom on March 16. Though the charge against her is second-degree murder, it may be reduced to infanticide, which carries a maximum punishment of five years in adult court or two years in youth court. Like Elizabeth Rainer, the Brampton girl (she cannot be named under the Young Offenders Act) will be disgraced if convicted. Still, she'll likely be out of jail before her 18th birthday.

There is something shocking about this leniency. In many ways, baby-killing is worse than ordinary murder. No human is more innocent or defenceless than a baby, or has more remaining years of life. And while our infanticide law speaks of a mind that is "disturbed," that low standard does not presume true insanity, or even a lack of free will. Why, then, does our law enshrine the belief that postpartum infanticide is a lesser crime than homicide?

Edward Hagen, a research scientist at the Institute for Theoretical Biology in Berlin, is one of the few scholars who has made a serious attempt to get to the root of this question. His research not only sheds light on our laws, it also suggests ways infanticide can be prevented.

Hagen follows the thesis that maternal infanticide is a product of the evolutionary pressures women experienced in ancestral times. Breast-feeding is, in metabolic terms, expensive: It eats up about 500 calories a day. This is not a problem for us, but it was for our hunter-gatherer ancestors, who often subsisted at near-starvation levels. "If parental investment can only occur at the expense of [self-maintenance], then parents need to decide ... whether it is more advantageous to invest finite resources in offspring, mates or themselves," wrote Hagen in the Journal of Evolution and Human Behavior in 1999. "During our evolutionary history, neglect and infanticide may have been two common forms of defecting from the child-raising venture." Under this logic, our law simply reflects an allowance for natural instinct.

(This analysis applies only to women. When men kill their children, it is often because they believe they've been cuckolded, their reaction being linked to the genetically learned aversion toward rearing unrelated children. Jay Handel, the B.C. man accused of killing his six children earlier this month, fits this pattern. Weeks before the tragedy, he accused his wife of having an affair.)

Severe postpartum depression of the type that afflicted Andrea Yates, Hagen theorizes, is a parallel evolutionary adaptation women learned so they could signal their need for more attention and resources: "In the same way that a valuable employee may attempt to negotiate a larger salary by threatening to quit, mothers receiving insufficient social support may attempt to negotiate larger levels of support by threatening to defect from (i.e., quit) the childrearing endeavor." Other experts provide similar explanation. "Depression [among First World women] is most severe," says evolutionary psychologist Stephen Pinker, "in the circumstances that lead mothers elsewhere in the world to commit suicide."

Elizabeth Rainer and the Brampton mother (if convicted) fit Hagen's model. (Yates's case is more complicated. Like most women who kill their older children, she is clinically insane.) Both had good reason to suppose they would have a disgraced, manless future if they kept their babies. And in some long-forgotten corner of their respective brains was the instinctively programmed belief that infanticide might lead to possibilities -- a husband, more resources, healthier children.

To explain is not to exculpate. But such analysis at least helps us understand behaviour that is too often written off with the single word "senseless." And it may help us head off such behaviour, too. Hagen's theories and the available empirical data support the view that severe postpartum depression and maternal infanticide are correlated with a lack of emotional and material support from fathers, families and friends. The best way to prevent tragedy, therefore, is to ensure such support is never denied.