About the SA Blog Network

The Primate Diaries

The Primate Diaries

Notes on science, politics, and history from a primate in the human zoo.
The Primate Diaries Home

Stressing Motherhood: How Biology and Social Inequality Foster Maternal Infanticide

The views expressed are those of the author and are not necessarily those of Scientific American.

Email   PrintPrint

"Mom" by Nathaniel Gold

          "Mom" by Nathaniel Gold

Chicago’s nineteenth ward reeked of overripe fruit and kerosene the day Mary Stastch killed her baby. According to the Chicago Tribune on July 29, 1911 the unemployed single mother and recent immigrant from Austria left Cook County Hospital earlier that week and “wandered about Chicago for two days with the baby in her arms, looking for work.” But with the growing labor crisis leaving nearly 250,000 people jobless her search would have been difficult even without a newborn in tow.

As if that wasn’t enough, the following day more than three hundred police descended on the largely immigrant neighborhood around Maxwell Street in what was described as “a day of rioting and wild disorder such has not been seen in Chicago since the garment workers’ strike” the previous year. Wagons were overturned, grocery store windows smashed, and fruit carts doused with fuel in a desperate struggle between peddlers, police, and strike breakers. In the eery silence that followed Mary Stastch quietly strangled her infant. Cradling the limp child in her arms she then carried the body several miles to where it was later discovered, hidden behind a residence on Carroll Avenue.

“Cases of maternal infanticide are gripping,” explains feminist scholar Rebecca Hyman, “because they seem to violate an inherent natural law.” A mother’s affection for her child is thought to be absolute, a fact of evolution in which women have been “endowed with a nurturing maternal instinct.”

Yet, throughout history, from the fictional Medea to the tragic reports of modern times, women have taken the lives of their children under a variety of contexts, whether it is to punish the father, escape from the burden of motherhood, or even to protect a child from what they perceive as a fate worse than death. In this regard humans share yet another feature, albeit a tragic one, with nonhuman animals since females in a variety of species have been observed to abandon, abuse, or even kill their own offspring. To stress the importance of motherhood in human societies today, how can we best understand this behavior so that we can better predict, and prevent, its recurrence?

One hundred years after Mary Stastch took her child’s life another Chicago immigrant may have some answers. Dario Maestripieri has spent most of his career studying maternal behavior in primates. In particular, he’s focused on the factors that influence a mother’s motivation towards her young. As a professor of Comparative Human Development, Evolutionary Biology, Neurobiology, and Psychiatry at the University of Chicago he has enjoyed the kind of cross-disciplinary success that most scientists only dream of. His 153 academic papers and six books have been cited more than a thousand times by scholars (including this one) in many of the world’s top scientific journals. His latest paper was published in the June, 2011 edition of the American Journal of Primatology. In it Maestripieri lays out the argument he’s built over the last two decades showing how one of the most serious impacts on maternal behavior, one with potentially lethal results, is so common in modern life as to be nearly invisible: stress.

Of course, contrary to the advice of most doctors, stress is actually a good thing and is particularly adaptive during motherhood. Whenever animals experience a stressful situation, whether that involves chasing down a gazelle, escaping from a hawk, or asking a cute guy out on a date, our adrenal gland releases massive amounts of the hormone cortisol into our blood stream. Cortisol, in turn, increases the production of glucose and aids in metabolizing fats, proteins, and carbohydrates for even more blood sugar. Within moments we have enough energy on hand to attack or flee depending on the circumstances. Stress therefore serves as an adaptive response to prepare the body for adversity.

Both during pregnancy and after parturition cortisol increases significantly for new and experienced mothers alike. This suggests, to no one’s great surprise, that motherhood is an especially stressful time in a female primate’s life. Dario Maestripieri has previously shown that this response is directly related to protective behaviors that keep a mother’s infant from harm. For example, in one study, rhesus macaque mothers who exhibited high levels of anxiety (such as repeated self-scratching, a behavior associated with high cortisol levels) while they were observing their infant near a dangerous group member, were much more likely to immediately intervene and retrieve them.

“Maternal anxiety,” explains Maestripieri, “can significantly increase the chances of offspring survival and the reproductive success of the parent.” Natural selection has provided mothers with an early warning system, one that can alert them to danger before others are even aware of the risk.

However, as the maxim goes, there can be too much of a good thing. In addition to increasing the body’s available energy, cortisol also serves to inhibit other systems, such as digestion or immune function, that can be spared over the short term. The reason why, as Stanford neuroscientist Robert Sapolsky quipped in his book Why Zebras Don’t Get Ulcers, should be relatively straightforward:

“You have better things to do than digest breakfast when you are trying to avoid being someone’s lunch.”

But periods of long term or excessive stress can cause serious physiological damage and an increased susceptibility to disease. It can also result in what Maestripieri calls the “dysregulation of emotion,” or turning what would be an otherwise adaptive response into a potentially dangerous overreaction.

“A large body of evidence,” Maestripieri says, “indicates that extremely high or chronically elevated cortisol levels due to stress can impair maternal motivation and result in maladaptive parenting behavior.”

Maestripieri has conducted numerous studies demonstrating the connection between high levels of stress and maladaptive parenting. For example, his research has shown that, among pigtail macaques, maternal abuse of infants is frequently precipitated by socially stressful events. Likewise, he’s found that abusive rhesus macaque mothers have neurochemical profiles similar to those of humans with posttraumatic stress disorder.

“Specifically,” Maestripieri says, “it has been shown in humans that stress is a major risk factor for postpartum depression and for child neglect and abuse.” But his latest findings are the most revealing yet about how stress and motherhood can interact in ways that are frighteningly relevant for today’s society.

While its appearance suggests an idyllic utopia of crystal blue waters, palm trees, and white sandy beaches, the island of Cayo Santiago is actually a breeding ground for class warfare. For more than seventy years this Caribbean island has been home to a provisioned colony of rhesus macaques, one that offers the ideal conditions to study the effects of stress and group dynamics.

Average change in cortisol levels for pregnant/lactating females in three social ranks.

Average change in cortisol levels for pregnant/lactating females in three social ranks. Reproduced from Hoffman et al. (2010).

Over a period of two years Maestripieri and his doctoral student Christy Hoffman studied the cortisol responses among 70 females, all of whom were experienced mothers. Blood samples were regularly collected from each individual and behavioral data were recorded to determine the dominance hierarchy among the mothers. The study confirmed earlier results showing heightened reactions to stress for all mothers from conception through the end of weaning. However, the largest change in cortisol levels occurred among the lowest ranking females and was four times greater than those who were higher in the pecking order. The most likely explanation for this, say the scientists, was a lack of control.

“Low-ranking mothers may perceive their infants to be at risk from other group members to a greater extent than middle- and high-ranking females,” says Hoffman. However, unlike the higher ranking females, these low status mothers “experience greater constraints in their ability to provide protection for offspring.”

Supporting this interpretation, the team analyzed the colony’s mortality records covering a period of ten years and found that infants born to low-ranking females were much more likely to die in their first year than those born to high-ranking ones. As a result, low-ranking mothers were living in a state of constant panic. Resources were harder to come by and there was danger at every turn, for themselves but especially for their offspring. Unable to control the social forces around them their innate warning system screamed at high alert and their anxiety simply grew, expanding out of proportion as a result of the social inequality. This created an environment with a clear link to abuse, neglect, and even infanticide.

But are such findings applicable for our species? After all, humans have the ability to make conscious choices and design political systems that protect the least among us. Haven’t we improved on the harsh conditions faced by our distant monkey cousins? The answer to this couldn’t be more clear: humans are very different from macaques. We’re much worse. The anxiety caused by human inequality is unlike anything observed in the natural world. In order to emphasize this point, Robert Sapolsky put all kidding aside and was uncharacteristically grim when describing the affects of human poverty on the incidence of stress-related disease.

“When humans invented poverty,” Sapolsky wrote, “they came up with a way of subjugating the low-ranking like nothing ever before seen in the primate world.”

This is clearly documented in studies looking at human inequality and the rates of maternal infanticide. The World Health Organization Report on Violence and Health reported a strong association between global inequality and child abuse, with the largest incidence in communities with “high levels of unemployment and concentrated poverty.” Another international study published by the American Journal of Psychiatry analyzed infanticide data from 17 countries and found an unmistakable “pattern of powerlessness, poverty, and alienation in the lives of the women studied.”

The United States currently leads the developed world with the highest maternal infanticide rate (an average of 8 deaths for every 100,000 live births, more than twice the rate of Canada). In a systematic analysis of maternal infanticide in the U.S., DeAnn Gauthier and colleagues at the University of Louisiana at Lafayette concluded that this dubious honor falls on us because “extreme poverty amid extreme wealth is conducive to stress-related violence.” Consequently, the highest levels of maternal infanticide were found, not in the poorest states, but in those with the greatest disparity between wealth and poverty (such as Colorado, Oklahoma, and New York with rates 3 to 5 times the national average). According to these researchers, inequality is literally killing our kids.

Did the stress associated with inequality also play a role in the death of Mary Stastch’s child? We can’t ever know what this young woman was thinking or feeling at the time but, according to Michelle Oberman who documented Mary’s story, it would have been an important factor.

“She would have been desperately in need of food, clothing, shelter, and money,” says Oberman. “Indeed, it was all but inevitable that harm would befall that child.”

While the ultimate explanation for Mary Stastch’s murder must remain stubbornly obscure from us, the conditions that give rise to her modern counterparts around the world are starting to become more clear. Even though macaques shared a common ancestor with humans more than 25 million years ago, they may have something very important to teach us about the world we live in today.

As social mammals, primates are powerfully affected by their standing in a given society. Even a bond that is as integral as that between a mother and her child can be severed if social conditions are pitted against her. If we’re serious about stressing motherhood in our society it will take more than simply reaching out to at-risk populations. It will require addressing the root cause of societal ills and preventing those risks before they start.


Friedman, S. (2005). Child Murder by Mothers: A Critical Analysis of the Current State of Knowledge and a Research Agenda, American Journal of Psychiatry 162 (9), 1578-1587. DOI: 10.1176/appi.ajp.162.9.1578

Gauthier, D., Chaudoir, N., & Forsyth, C. (2003). A Sociological Analysis of Maternal Infanticide in the United States, 1984-1996, Deviant Behavior 24 (4), 393-404. DOI: 10.1080/713840226

Hoffman, C., Ayala, J., Mas-Rivera, A., & Maestripieri, D. (2010). Effects of reproductive condition and dominance rank on cortisol responsiveness to stress in free-ranging female rhesus macaques, American Journal of Primatology 72 (7), 559-565. DOI: 10.1002/ajp.20793

Maestripieri, D. (2011). Emotions, stress, and maternal motivation in primates, American Journal of Primatology 73 (6), 516-529. DOI: 10.1002/ajp.20882

Oberman, M. (2002). Understanding Infanticide in Context: Mothers Who Kill, 1870-1930 and Today, The Journal of Criminal Law and Criminology 92 (3/4), 707-738. DOI: 10.2307/1144241

Sapolsky, R. (1998). Why Zebras Don’t Get Ulcers. New York: W.H. Freeman and Company.

Originally published on the SA Guest Blog

Eric Michael Johnson About the Author: Eric Michael Johnson has a Master's degree in Evolutionary Anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics.

Follow his work on Facebook and Google+. Follow on Twitter @primatediaries.

The views expressed are those of the author and are not necessarily those of Scientific American.

Rights & Permissions

Comments 10 Comments

Add Comment
  1. 1. killgrove 12:55 pm 07/22/2011

    This was a really great post; a nice counterpoint to the normally male-focused discussion of infanticide in primates. As the discussion about postpartum depression grows and the condition becomes more understood, it seems it’s not just biological, but also a result of changing social norms – e.g., the lack of extended family living nearby, the fact that we have many fewer offspring than centuries ago, which can combine to make a new mother feel isolated, anxious, and unsure of herself. I think it’s important for the public at large to understand the contributions that social conditions make to parenting choices, because motherhood (and fatherhood) isn’t as simple or romantic as looking into your newborn child’s eyes and falling in love.

    Link to this
  2. 2. EricMJohnson 1:22 pm 07/22/2011

    I couldn’t agree more killgrove. Thank you for your comment. Coincidentally, a BBC Health story just came out this week that described new research showing how a mother’s stress can be passed to her baby in the womb and have long lasting effects:

    It makes me wonder if inequality (with its known links to stress-related illness and emotional dysregulation) should be treated as a public health issue?

    Link to this
  3. 3. vagnry 2:41 pm 07/22/2011

    I completely agree with killgrove, immediately thought of a linkage to postnatal (-partum) depression.

    It would be interesting to see, if there is a similar linkage between mothers’ infanticides and social inequality, for instance as expressed by Gini-curves of different nations with approximately equal GDP (PPP) per capita.

    Link to this
  4. 4. Kristina_Killgrove 3:30 pm 07/22/2011

    I just checked a leading bioanth textbook, and it notes that “Infanticide is a sexually-selected male reproductive strategy” (citing Sarah Blaffer Hrdy’s work). I’m curious, Eric, whether you would go so far as to say that maternal infanticide is a reproductive strategy? Is it a survival technique? Are the two mutually exclusive? I’m always a bit wary of sexually-selected traits in primates, so I’m genuinely curious about this topic.

    Link to this
  5. 5. EricMJohnson 4:19 pm 07/22/2011

    That is a really excellent question. Maestripieri certainly doesn’t make that claim. The best answer right now is that this is unknown. There have only been a handful of observed cases of maternal infanticide in the wild. The most recent paper was published this year in Primates (Culot et al., 2011). They documented two cases in the polyandrous Callitrichid species known as moustached tamarins (Saguinus mystax). Of Hrdy’s (1979) five hypotheses for infanticide they rejected all but one: parental manipulation. They suggested that “the mother could have terminated the investment in offspring that had low chances of survival in a group with low availability of helpers.” I linked to another case above in black fronted-titi monkeys but the authors weren’t able to settle on a single hypothesis to explain the behavior.

    I would say that it’s possible, though still untested, that maternal infanticide could be adaptive in some circumstances for primates. In a similar process as that described by life history theory, in which primate females will alter their reproductive cycles and levels of proceptivity based on environmental conditions, maternal infanticide could be a way to conserve resources for a time and place that was more conducive to raising viable offspring. Under this hypothesis an over-the-top stress response could be the mechanism promoting this strategy. But given that neglect and abuse are more common behaviors under conditions of high stress it doesn’t seem to be a very specific or effective mechanism. Until there’s better evidence I would side with Maestripieri’s explanation of emotional dysregulation.

    Link to this
  6. 6. mghertner 4:43 pm 07/22/2011

    The author makes an international comparison between the U.S. and the developed world about the rate of maternal infanticide, but when discussing the influence of relative inequality the author switches to a domestic comparison between individual U.S. states. WTF?

    Why only look at the developed world? If there is in fact a causative connection between relative inequality and maternal infanticide, wouldn’t we expect developing countries – like those in South America and Africa with a higher Gini coefficient than the U.S. – to have an even greater maternal infanticide rate? Perhaps they do, but only looking at developed countries won’t give us the answer.

    Also, the article begins with a case study of an immigrant and an immigrant community. Immigration of poor people to rich countries does indeed increase the level of inequality as measured in that country alone, but at the same time decreases inequality overall as measured internationally. No doubt being a poor, unskilled immigrant surrounded by much wealthier neighbors sucks, but revealed preference indicates that the immigrant would rather be here than there. Further, if we are looking at this from only a health perspective, we would need to compare the health risks to the immigrant of residing in the U.S. (a higher rate of maternal infanticide, possibly) to the health risks of residing in their native country (disease, higher absolute poverty, etc.)

    Throughout the article the author uses relative inequality and absolute poverty interchangeably. No one would dispute that poverty is stressful, but when claiming that inequality alone increases stress levels – above and beyond the increased stressed levels caused by poverty – it is disingenous to equivocate beween the two. Surely it is more stressful for a single mother to be “desperately in need of food, clothing, shelter, and money” than whatever stress results from keeping up with the Joneses. If so, than it is preferable to have greater inequality with corresponding lower poverty than to have greater equality and higher poverty.

    Finally, to whatever extent inequality leads to unhappiness, the question of whether to ameliorate that unhappiness itself depends on the legitimacy we place on the underlying cause. Jealousy is generally thought of as a negative attribute to be discouraged, not a justification for satisfying the desires of the jealous. Even if it were the case that ending Jim Crow caused a net amount of unhappiness – perhaps the number of white racists was much greater than the number of subjugated blacks – we still would not say it is legitimate to satisfy the desires of racists at others’ expense.

    Link to this
  7. 7. kristi276 5:46 pm 07/24/2011

    The human kicks the dog. The dog kicks the cat. The cat kicks the mouse. The mouse kicks the roach. The roach kicks the ant, but the ant has no one to kick.

    Stress and social inequality is the stratification of society into the various class components of society, the laborers, the middle class to the rich (the ruling class). Gender inequality (Women’s inequality) is, also, inherent in the stratification of society based on those social layers, but the blunt of inequality lies in the lower class of society; that being of the working class.

    It has been over one hundred years since the Women’s suffragist and the abolition movements of the 1800′s, and still women suffer form inequality in many segments of economic and social life. In 1851 Sojourner Truth spoke at the Women’s Rights Convention in Ohio, “ain’t I a women too”. Racial inequality stems from, not Jim Crow (the Black Codes) but from the issue of Chattel Slavery that brought about racial inequality. Although, native people still suffer from the Reservation system of segregation, Native women do not constitute a statistic in this study. How are Native women affected by racial and gender inequality; or are Native people not worth the mention? As we are aware of the problems, what are the solutions?

    Link to this
  8. 8. daedalus2u 8:41 pm 07/24/2011

    I have written about the physiology of infanticide in the postpartum period. It is absolutely about physiology and especially about the metabolic capacity to sustain lactation until the infant is weaned. Lactation is extremely energy intensive for mammals. Infanticide in the face of insufficient metabolic resources is something that essentially all mammals exhibit. They have to. Mammals have to have a mechanism to shed an unsustainable metabolic load.

    If a mother doesn’t have the metabolic resources to sustain lactation until the infant is weaned, then the child will die. A mother’s “best” reproductive choice under those circumstances is to kill the infant, survive herself, and reproduce another time. This is also the infant’s best reproductive choice. It is better to die and have a mother and future siblings survive than to die and have your mother die with those future siblings unborn.

    Not to condone maternal infanticide, but to understand it so it can be prevented before postpartum women develop postpartum psychosis and become infanticidal.

    There is cross-talk between other stress pathways and the insufficient metabolic capacity to sustain lactation. I don’t think it is the “stress” per se, rather the stress changes the priority of energy allocation to where lactation has a sufficiently low priority that the lactation metabolic load must be shed.

    It is carbohydrate that is most necessary for lactation, carbohydrate for gluconeogenesis to make lactose which is the major osmolyte in milk. If there is insufficient liver capacity for gluconeogenesis, then sufficient milk cannot be produced.

    Depot fat is useless for producing blood glucose, which is why obesity actually makes lactation more difficult. I go into the details of this physiology in my blog post.

    I have a way to fix it, by improving the regulation of energy status by inducing mitochondria biogenesis.

    Link to this
  9. 9. daedalus2u 1:59 pm 07/25/2011

    I just came across an interesting paper. Apparently maternal depression during early infant life is strongly associated with underweight (odds ratio 12.12).

    My explanation is that postpartum depression is the first sign of metabolic insufficiency. During maternal depression, infants then self-select a reduced caloric intake to reduce maternal metabolic stress. The reduced weight gain in the first year is made up essentially completely in the second year (which would be after weaning) as shown in figure 2.

    Link to this
  10. 10. Mythusmage 12:27 am 07/26/2011

    There is one factor to consider here, to what degree does the person, human or non-human, have control over his environment? What adaptive behavior is available for gaining and maintaining some degree of control. Or, rather, to what degree does the person see himself as being in control, of having options.

    People who see themselves as having no effective control over their lives will behave in a self-destructive way. When you see yourself as having no options, no control, you’ll behave in a manner that harms you. An act of desperation comes easier for people who know of no way to change their circumstances.

    Link to this

Add a Comment
You must sign in or register as a member to submit a comment.

More from Scientific American

Email this Article