Why do some mothers commit infanticide?

It’s reasonably hard for many to conceive how on earth could a mother ever decide to take the life of her own baby. After all, we’ve all been told that motherhood is divine and mother’s love unconditional and sacrosanct.

Yet, it’s a fact of life that infanticide happens and, as you’ll find out when you’re done reading this article, can’t be simply dismissed as ‘pathological’.


But first, shouldn’t infanticide be non-existent as per evolutionary logic? Isn’t it in the best interest of mothers that they ensure the survival and reproductive success of their young?


When mama bear kills baby bear


Many animals, including mammals, have been observed killing their own young and some even devour them after killing them. This includes mother bears, primates, felines, canids, rodents, insects, fish, amphibians, reptiles, and birds.


When a mother bear gives birth, she has to feed and nurture the cub. If she’s not well-nourished herself, she can’t feed the cub. Also, if the cub is unhealthy or deformed, it makes no sense to waste time and resources on an offspring who’s destined to die soon or may even fail to reproduce.


So, she kills her cub because she can always have other, possibly healthier and undeformed cubs in the future when her circumstances improve. 

When she’s under-nourished, unable to find food and her own survival is at stake, she might as well eat the cub she just killed. 


The clear pattern that emerges from all of this is that whenever a mother assesses her current circumstances as ‘unfavourable’ for feeding, nurturing, and raising her baby, her best course of action, evolutionarily speaking, is to get rid of the baby. 
Later, when circumstances improve, she can have offspring whose chances of survival and reproduction will be comparatively higher.

So when mammals kill their young during unfavorable circumstances, they’re actually increasing their chances of reproductive success.

Infanticide and parental investment in humans
What types of human mothers are more likely to kill their own young?

When the child is very young, the investment of parents is minimum and so the child is highly replaceable and less valued. As the child grows, the parental investment in terms of resources typically increases right up to the reproductive age and beyond.

When the child reaches reproductive age, she’s highly valued and irreplaceable. Also, by this time the mother may have passed the fertile phase of her life and thus might be unable to have and raise more children.

Considering all this, we can predict that infanticide by mothers should be frequent when the child is very young and also when the mother herself is young. Also, since unmarried women are likely to have less access to resources, they should indulge in the act more than those who are married to resource-providing partners.


Studies have shown that most cases of infanticide happen in the first few years of life and that younger, unwed mothers are more likely to commit the act than older, married mothers.1


The case of Susan Smith

In 1994, Susan Smith (then 23) drowned her own kids- two boys aged 1 and 3, killing them. A few days before her murder, a high-status man she was having an affair with broke off the affair because he didn’t like the fact that she had children. Also, her marriage with her low-status husband had been tumultuous.

When she committed the crime, her mind must have done this evolutionary calculus, albeit unconsciously:

“If I get rid of my young children that I had with my husband, I can better my prospects of attracting another high-status man with whom I can have children in better conditions.”

susan smith killer mother
In a letter to a newspaper from prison, Susan Smith said that she didn’t plan the act and wasn’t in her ‘right mind’ when she committed the crime.

Unconscious family planning


Since parenting is very costly for human females, they’ve evolved a number of unconscious strategies to avoid having a child in unwanted circumstances and these strategies begin long before the actual birth.2

For instance, circumstances may be favorable when a woman ovulates but no longer so by the time the egg reaches her womb. In this case, her body might avoid implantation- the egg fails to attach to the womb.


Similarly, circumstances may be favorable at the time of implantation but may become unfavorable a month or so later.  In such a scenario, she might miscarry.


Even if conditions remain favorable throughout early pregnancy, they may still deteriorate towards the end of the pregnancy period, before the baby is born.


Interestingly, during the last three months of pregnancy, the woman’s psychology can undergo noticeable changes. There may be spells of ‘nest-building’ i.e. strong urges to prepare the upcoming baby’s environment. Also, there may be spells of intense reappraisal- primary targets being her partner, home, and general environment. 

These spells can manifest as phases of worry, anxiety, irritability, or even depression. This type of depression has been called prenatal (before birth) depression.


Again, if circumstances are favorable when the baby is born but worsen soon afterward, the woman may experience what is known as the postnatal (after birth) or postpartum depression. It’s usually accompanied by an irresistible urge to abandon, abuse, or kill a new baby.

A significant number of cases of post-natal depression are observed in women from low-income families. A study showed that women of low socioeconomic status are 11 times more likely to display symptoms of prenatal (before birth) and postpartum depression.3

So if a woman works at a low-paying job and her husband loses his job right after her baby is born, there’s a high chance she might experience postpartum depression and feel the urges to abandon or kill her baby.


References:

1. Bridgeman, B. (2003). Psychology and evolution: The origins of mind. Sage.

2. Baker, R. (1996). Sperm wars: Infidelity, sexual conflict, and other bedroom battles. Basic Books.

3. Goyal, D., Gay, C., & Lee, K. A. (2010). How much does low socioeconomic status increase the risk of prenatal and postpartum depressive symptoms in first-time mothers?. Women’s Health Issues20(2), 96-104.
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