A primatologist discovers the social factors responsible for maternal infanticide

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

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 350 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.

A mother killing her child challenges our understanding at the same time that it rips at our emotions. “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."

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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 is scheduled to be published in early 2011 by 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 for 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 the Stanford neuroscientist Robert Sapolsky quipped in his book Why Zebras Don’t Get Ulcers, should be relatively straightforward.

"You have better things to do," Sapolsky wrote, "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. Other studies have found comparable results in our own species.

"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.

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. They would watch as their offspring were confronted by dangerous group members but they were powerless to do anything about it. Unable to act while their innate warning system screamed at high alert, their anxiety simply grew, expanding out of proportion as a result of the social inequality.

While this latest study on Cayo Santiago did not look at maternal abuse, previous work has shown a clear link between infanticide and extreme cases of maternal stress. 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 seen 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 humans and macaques shared a common ancestor 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.

UPDATE: David Dobbs at Wired has an excellent discussion inspired by this post and raises some important questions worth considering:

I agree with what we might call the social lessons Johnson draws from these studies. But I wonder if stopping there pulls us up short of some of the more interesting scientific implications of these studies. I wonder if stopping there mires us in a vision of environment that gives it too much primacy. It suggests that environment is deterministic.

References:

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. (2009). Effects of reproductive condition and dominance rank on cortisol responsiveness to stress in free-ranging female rhesus macaques. American Journal of Primatology DOI: 10.1002/ajp.20793

Maestripieri, D. (1994). Infant abuse associated with psychosocial stress in a group-living pigtail macaque (Macaca nemestrina) mother. American Journal of Primatology, 32 (1), 41-49 DOI: 10.1002/ajp.1350320105

Maestripieri, D. (1995). Assessment of danger to themselves and their infants by rhesus macaque (Macaca mulatta) mothers. Journal of Comparative Psychology, 109 (4), 416-420 DOI: 10.1037/0735-7036.109.4.416

Maestripieri, D. (2000). Causes and consequences of infant abuse and neglect in monkeys. Aggression and Violent Behavior, 5 (3), 245-254 DOI: 10.1016/S1359-1789(98)00019-6

Maestripieri, D. (2010). Emotions, stress, and maternal motivation in primates. American Journal of Primatology DOI: 10.1002/ajp.20882

Maestripieri, D., Lindell, S., Ayala, A., Gold, P., & Higley, J. (2005). Neurobiological characteristics of rhesus macaque abusive mothers and their relation to social and maternal behavior. Neuroscience & Biobehavioral Reviews, 29 (1), 51-57 DOI: 10.1016/j.neubiorev.2004.05.004

Oberman, M. (2002). Understanding Infanticide in Context: Mothers Who Kill, 1870-1930 and Today. The Journal of Criminal Law and Criminology (1973-), 92 (3/4) DOI: 10.2307/1144241

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

Image Credits: Medea prepares to kill her children. Eugene Delacroix (1862). Painting of Medea courtesy of Wikimedia Commons.

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 the University of British Columbia looking at the interplay between evolutionary biology and politics. His work has appeared in the Journal of Human Evolution, the American Journal of Physical Anthropology, Discover, Seed, Psychology Today, Wildlife Conservation, and The Huffington Post.

He blogs at The Primate Diaries and can be found on Facebook and Twitter: @ericmjohnson. [Image used with permission by Ben Jones Photography]

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