Parental investment and life history theory approaches to child nutrition and growth
In small, kin-based communities, are some children cared for better than others? Further, within families, do parents invest differently in different children, and, if so, why? My research addresses the behavioral and psychological aspects of this question using data from both rural Ecuadorian Shuar and middle class North American populations.
The Shuar are Native South American horticulturists. In my most recent study, anthropometric and other health data were collected from approximately 150 members of a Shuar village in Ecuador. Indices of recent access to food included body mass index (BMI), and an index of body fat computed from triceps and abdominal skinfold thicknesses. Indices of long term access to food and other nutrients were height-for-age, arm muscle area, and a 'development' index computed from mid-upper-arm circumference and calf circumference. Family variables that were predicted to correlate with these indices included the number of consumers, the number of producers, reproductive value, and father status. After controlling for age and sex, each of these variables was significantly correlated with the indices of recent access to food among dependents aged 3-20, and a multivariate regression model incorporating all four accounted for 38% of the variance in the body fat index, and 29% of the variance in BMI. The indices of longer-term access to food and nutrients correlated with number of consumers and producers, and father status for this same group of dependents. A multivariate regression model incorporating these three variables accounted for 23% of the variance in the development index. The consumer/producer ratio was correlated with arm muscle area. These results accord well with parental investment theory, as well as with the work of Chayanov.
An evolutionary medical approach to depression and suicidality
A quiet revolution is underway in the biology of animal behavior and cognition that could also transform our conceptions of human mental health. Many biologists now believe that the process of natural selection and that of rational decision-making converge to very similar solutions: Animal and plant strategies that evolved by natural selection look as if they are based on the rational decision-making predicted by economic theory, where the utility that is maximized is biological fitness. Ironically, as animal and plant biologists eagerly embrace economic theory, economists and psychologists increasingly reject the basic economic assumption that people think and behave rationally. Countless experiments with humans have revealed seemingly profound errors in reasoning, and, in certain circumstances, a persistent refusal to behave in a manner that would maximize profit--humans are not Homo economicus. These findings have recently earned their discoverers Nobel Prizes in economics.
I will first briefly explain how the apparent contradiction between biologists' increasing enthusiasm for economic theory, and economists' and psychologists' increasing skepticism, is resolved (humans are animals after all). This resolution has been used to provide a concept of mental disorder that has strong links with modern evolutionary theory. I then explain that an ability to choose social partners leads to behavior that corresponds to the economic theory of markets, whereas an inability to choose leads to behavior that corresponds with the economic theory of bargaining. I conclude by sketching how the synthesis of the economics of bargaining with Darwinian theory leads to a radically different conception of suicidality and Major Depressive Disorder.
How economically irrational behavior is ecologically rational
Many biologists and social scientists, including one of the aforementioned Nobel Prize winners, argue that apparent errors in human reasoning and decision-making can be seen to be 'rational' under the ecological conditions in which humans evolved. In naturalistic environments with stereotypical features, where information, time, and brainpower are limited, supposedly irrational decision processes can be reinterpreted as simple, effective heuristics that will usually yield good, if not perfect, results. Humans, like other animals, should be ecologically rational.
Yet Homo ecologicus, properly construed, is not simply a dumber Homo economicus. If animals forage like economic optimizers and exchange goods and services like market traders, then it is not because they are general utility maximizers consciously making rational decisions, nor is it because they reason using simpler but still generic heuristics. It is because they have highly specialized psychological mechanisms that evolved to enable these particular strategies. In humans, ecologically rational strategies are grounded, not in the cold rationality of Homo economicus, but in evolved mechanisms that may sometimes be experienced as anything but 'cold' or 'rational.'
It is one of the deepest of ironies that the human traits that have long been viewed as the antithesis of the rational--hot emotions like anger, fear, and jealousy--may be the product of a process, natural selection, that converges to the same solution as rational decision-making. The evolutionary game theory of conflict and parental investment predicts anger and jealousy. These emotions make economic sense in light of a human evolutionary history of limited access to resources like food and mates.
Ecological rationality and the harmful dysfunction concept of mental disorder
The unexpected synthesis of Darwinian evolution and economic theory forged over the last decades has important implications for human mental health. Currently, 'normal' psychology is operationalized in the DSM and most mental health research as a statistical norm. Whatever most people feel and do is 'normal'. Abnormal psychology is then whatever is not 'normal', usually with the added criterion that the deviant experience be considered harmful to the sufferer.
It is increasingly recognized that this operationalization of 'normal' and 'abnormal' psychology is inadequate. Consider the following example. Nausea is an aversive and relatively rare experience (i.e., most people do not experience it most of the time). Yet it is rightly not considered 'abnormal' because it has clear utility. Nausea would have protected our ancestors from frequently encountered toxins. Nausea, a suite of rapid and powerful physiological and psychological changes, is not 'normal' in the statistical sense; rather, it is ecologically rational. Abnormal psychology should therefore be defined as harmful dysfunction (Wakefield 1992)--the dysfunctioning of evolved psychological mechanisms that are harmful to the patient. Thus, a failure to experience nausea under appropriate circumstances would be a disorder.
If this view is correct, mental health cannot be defined as a statistical norm, nor can it be defined as mental experiences that are perceived as harmful. Instead, psychological states must be tested for their ecological rationality: would they or would they not have provided biological fitness benefits in the social-ecological conditions under which humans evolved? If so, such states are not disorders, even if they are aversive and relatively rare.
Wakefield and others have argued that the modern evolutionary biological perspective sketched here best captures mental health professionals' intuitions of a 'mental disorder' and that it would provide psychiatry with a solid conceptual and scientific framework for the 21st century. Unlike most who so argue, I believe it might also completely overturn our conceptions of some psychiatric phenomena, like Major Depression.
Market theory vs. bargaining theory
The ability to choose partners can have a profound impact on social relationships. Market theory applies when 'consumers' have choices. Marine reefs harbor a rich variety of fish species. Some species of reef fish (cleaner fish) have evolved to clean other fish species (clients) by removing and eating their ectoparasites. Cleaner fish have 'stations' on the reef where client fish present themselves to be cleaned. Client fish can choose to be cleaned at one of a number of cleaner stations, whereas cleaners can vary their level of 'service.' Careful study of cleaner and client fish has revealed that their behaviors closely correspond to predictions of the economic theory of markets. Just as the price of gas is partially set by competition among gas stations, the service levels provided by cleaner fish correspond to 'market conditions' on the reef in terms of the ability of clients to choose another cleaning station--as the choice of clients increases, so, too, does the level of service provided by cleaners.
Choice is limited or absent, however, in many categories of human cooperation. A father with a new baby cannot simply choose the best mother for it in a 'mother market.' He and the mother are more or less stuck with one another. Similarly, children cannot choose the best parents on a 'parent market.' Choice may have been even more limited in ancestral human hunting and gathering groups. Kinship-based social organization, intergroup conflict, and low population densities would have limited the ability of ancestral humans to choose cooperative partners in a market. Even choice of mates would have been limited because families often arranged marriages.
In markets, the power of individuals to set the terms of exchange resides in their ability to choose among partners--if there is a conflict over 'price,' choose another partner. When partner choice is limited or absent, however, economic power resides instead in the ability to withhold benefits until better terms are forthcoming--that is, individuals can 'go on strike.' Labor and management can realize substantial profits by cooperating, but there is a conflict of interest over the division of these profits. When management cannot choose workers from a market, and laborers cannot easily switch companies, the division of profits must be determined by bargaining (as economists term this type of negotiation tactic). Should laborers conclude that they are not being paid enough, they can go on strike, withholding the benefits of their labor until better salaries are offered.
If, over evolutionary time, choice was limited in many domains of human cooperation, if there was conflict in ancestral human groups over the division of benefits, and if organisms have evolved to be ecologically rational, then modern biology strongly predicts that humans should have evolved a psychology for bargaining--'going on strike.'
Applying bargaining theory to Major Depression
Recall that ecological rationality is not the cool, conscious rationality of H. economicus, but is instead often realized in powerful and distressing emotions like jealousy that override conscious decision-making. Nor does ecological rationality merely intrude at the margins of decision-making. Life is a game that is played for keeps, and the stakes can be very high indeed. An evolved psychology for 'going on strike' could therefore involve powerful emotions and other psychological states beyond conscious control that ruthlessly prevent an individual from providing benefits to social partners. These states would be triggered when partner choice is limited and when an individual needs her social partners to provide more benefits (or make decisions that would benefit her), but her social partners are reluctant to do so. I propose that major depression (MD) might be just such a strategy--MD might not be an illness, but an ecologically rational aspect of human psychology.
Parenting is one cooperative domain where individuals have little choice in social partners, and where 'going on strike' could be effective in establishing equitable levels of investment in a new offspring. A nursing infant consumes about 500 kcals/day from its mother. In ancestral foraging groups, this energetic cost would have been unaffordable without substantial provisioning by the father or other family members. The provisioning, however, might not always have been provided. If the father had children by another wife, he might prefer to provision them instead. Or the father might prefer pursuing other women instead of investing in an infant. If sufficient provisioning were not forthcoming, a mother could bargain by withholding her investment in the infant until assistance by the father or others was sufficiently increased. In the provided paper, I show that postpartum depression (PPD)--a major depressive episode with postpartum onset--is closely associated with, and probably caused by, circumstances in which the mother's costs are high and her benefits are low. I show that mothers with PPD significantly reduce their investment in offspring, and, in a study of mine (Hagen 2002) as well as two other independent studies, PPD has been shown to be associated with increased levels of investment by the father and other family members (these studies were correlational, so the direction of causation has not been established). These results support the bargaining model. Many facts about MD in general also support the bargaining model:
- MD is caused, in part, by severe negative life events like death or the loss of a relationship--exactly the circumstances in which individuals might need more help from others.
- MD has been shown to be closely associated with social conflict that might prevent others from providing needed assistance or making other social changes that would benefit the MD sufferer.
- A pervasive 'loss of interest in virtually all activities' is the hallmark of MD, and it results in dramatic reductions in productivity.
- MD causes a negative reaction in others, which is to be expected if MD is a strategy to compel increases in assistance by withholding important benefits (management usually does not welcome a labor strike).
- Countless studies have shown MD to be associated with 'hopelessness', 'helplessness', or 'entrapment', the feeling that one cannot unilaterally take the actions needed to improve one's life, and/or that one is 'blocked' by powerful others.
- Detailed observational studies of MD in both laboratory as well as naturally occurring settings reveals that despite the negative reaction they cause in others, depressive symptoms elicit help and support--so much so that clinicians worry that these benefits reinforce the depression!
- Increased social support and positive life changes predict MD remission.
Deficiencies of central noradrenergic and/or serotonergic systems have long been thought to be the physiological basis of MD, but this monoamine hypothesis faces a number of difficulties. Alternatively, there is rapidly increasing evidence that chronic activation of the HPA axis is a proximate cause of MD. Altered stress hormone secretions may be causally involved in the development and course of MD. Biological psychiatry interprets these alterations as dysfunctions of the stress system. What biological psychiatry may be discovering is not the dysfunctioning of the stress system, however, but the proper functioning of a mechanism that evolved to reduce productivity when an individual was suffering chronic stress induced by social needs that were unmet due to social conflicts with others.
Suicidality is intimately associated with MD. Under the bargaining model, suicidality just extends the proposed function of MD: it is a threat to permanently remove oneself as a source of benefits to others. If suicidal ideation and attempts usually resulted in a successful suicide, then they would best be understood as dysfunctions. But they don't. Suicidal ideation and attempts vastly outnumber suicide completions. A suicidal threat could therefore be ecologically rational, as long as, on average, the fitness benefits received from making the threat outweighed the fitness costs of successful suicides over many generations. Consistent with the bargaining model, family and friends commonly perceive suicide threats and attempts to be manipulative.
Because, in contrast to other MD symptoms, suicide threats and attempts are quite visible to anthropologists conducting routine fieldwork, there have been many ethnographic studies of suicidality. These studies paint a portrait of suicidality that closely resembles the bargaining model. Across a wide range of small-scale societies, suicidality is seen as an attempt by relatively powerless individuals to protest and redress perceived wrongs. A common example involves a young woman whose family has arranged a marriage she doesn't desire. Her suicide threats and attempts can thwart the marriage in favor of a more desirable match. Under the bargaining model, she is 'going on strike,' putting her value to her family at risk in an unconscious attempt to get a better deal. This interpretation is very similar to the indigenous interpretation.
Although ethnographic studies of depression itself are far fewer, they give the same picture. Depression is seen to be caused by perceived wrongs or frustrated desires, and it functions to redress these wrongs or help obtain what is desired by forcing community members to address the situation.
There is a cross-culturally robust 2:1 female bias in MD--women are much more likely to be depressed than men. The bargaining model provides several reasons for this bias, two of which are that (1) women may have less social power and be more likely to be exploited than men, and are thus more in need of a strategy to renegotiate social arrangements, and (2) whereas physical threats and aggression may be an effective strategy for men to compel social changes by powerful others, the relatively 'passive' MD may be a slower but less dangerous strategy to compel changes, and is thus more suitable for women.
Although negative life events are the most potent known cause of MD, negative life events do not always cause MD. The bargaining model easily explains this. MD is predicted only when there is a negative life event and there are social conflicts that make others reluctant to provide assistance. The bargaining model predicts an interaction between these two factors.
Conversely, many cases of MD are not caused by negative life events or any other identifiable social circumstances. This strongly challenges the idea that MD is ecologically rational. If it were shown that a large fraction of MD cases had no social cause, this would disprove the bargaining model. Despite the fact that not all cases of MD are caused by negative life events, it has not (yet) been shown that these cases have no social cause whatsoever. The bargaining model predicts that many social circumstances that are not thought of as negative life events should cause MD. The most obvious example is the birth of a new baby. This is not, a priori, a negative life event. For most women, in fact, it is a very positive life event. Like PPD, MD should be caused not just by negative life events, but by any situation where (1) individuals believe they can get more fitness benefits from cooperation, and (2) in order to obtain more fitness benefits they must convince others to change. This means that positive social circumstances could also be a cause of MD if taking advantage of those circumstances required others to change. For example, I found, as predicted, that men with more sexual opportunities in the postpartum period were more depressed than men with fewer (see Hagen 2002 for details). Thus, the bargaining model suggests that there are many unrecognized etiological factors for MD (for other potential challenges to the bargaining model, like genetic and personality factors and the kindling effect, and how the model would address them, see Hagen 2003).
The perspective sketched here does not imply that mental illness is a myth; there are many genuine mental disorders--schizophrenia, stroke related brain damage and Alzheimer's disease are but three examples. Nor does it prove my claim that MD is ecologically rational--MD could turn out to be, e.g., a dysfunction of sadness. I have long had a strong intuition, however, that MD is different. MD seems meaningful in a way that, e.g., Alzheimer's is not. MD is deeply grounded in life experience and circumstance. Indeed, therapies that involve talking about social conflicts and other life problems are as effective in treating MD as are the latest antidepressants. That would certainly not be the case with stroke-related brain damage or Alzheimer's.
If proven, the evolutionary model outlined here could significantly improve our ability to treat MD. Perceived lack of social support is an extremely well-established etiological factor for PPD, for example, yet very few studies have explored the efficacy of increasing social support as a treatment for it (those that have done so have generally found a positive effect). The current illness paradigm for PPD may be blinding psychiatry to the most obvious, common sense treatment of PPD: increase social support! Interpersonal Psychotherapy (IPT) rightly focuses on interpersonal problems, and is about as effective as anti-depressants in treating MD. The bargaining model explains why IPT works. With the deeper understanding of the social causes of MD provided by the bargaining model, much more could be done, however, to help sufferers resolve difficult conflicts with others and improve their lives.
Interviews and articles in the mainstream media that featured my research:
The Evolution of Depression – Does it Have a Role? All in the Mind ABC Radio National
We got rhythm; the mystery is how and why New York Times September 16, 2003.
Drug use linked to ancestors' habits New Scientist April 2, 2002.
Ancestors 'used drugs to survive' BBC News March 30, 2002
War of words New Scientist June 30, 2001.
Maternal instinct may be a killer National Post March 27, 2002
The dark side of parenting Hartford Advocate 2001
Mommy undearest Slate July 4, 2001
Postpartum misery tied to crash in hormone levels Toledo Blade June 28, 2001
Daughter's web site
