Biologists have known for decades that many traits involved in competition for mates or other resources and that influence mate choice are exaggerated, and their expression is influenced by the individuals' ability to tolerate a variety of environmental and social stressors. Evolution of Vulnerability applies this concept of heightened sensitivity to humans for a host of physical, social, psychological, cognitive, and brain traits. By reframing the issue entirely, renowned evolutionary psychologist David C. Geary demonstrates this principle can be used to identify children, adolescents, or populations at risk for poor long-term outcomes and identify specific traits in each sex and at different points in development that are most easily disrupted by exposure to stressors.
Evolution of Vulnerability begins by reviewing the expansive literature on traits predicted to show sex-specific sensitivity to environmental and social stressors, and details the implications for better assessing and understanding the consequences of exposure to these stressors. Next, the book reviews sexual selection-mate competition and choice-and the mechanisms involved in the evolution of condition dependent traits and the stressors that can undermine their development and expression, such as poor early nutrition and health, parasites, social stress, and exposure to man-made toxins. Then it reviews condition dependent traits (physical, behavioral, cognitive, and brain) in birds, fish, insects, and mammals to demonstrate the ubiquity of these traits in nature. The focus then turns to humans and covers sex-specific vulnerabilities in children and adults for physical traits, social behavior, psychological wellbeing, and brain and cognitive traits. The sensitivity of these traits is related to exposure to parasites, poor nutrition, social maltreatment, environmental toxins, chemotherapy, and Alzheimer's disease, among others. The book concludes with an implications chapter that outlines how to better assess vulnerabilities in children and adults and how to more fully understand how, why, and when in development some types of environmental and social stressors are particularly harmful to humans.
- Describes evolved sex differences, providing predictions on the traits that will show sex-specific vulnerabilities
- Presents an extensive review of condition-dependent traits in non-human species, greatly expanding existing reviews published in scientific journals, and more critically, extending these to humans
- Applies condition-dependent traits to humans to identify children, adolescents, or populations at risk for poor long-term outcomes
Chapter 1. Vulnerability
Chapter 2. Sexual Selection and the Evolution of Vulnerability
Chapter 3. Condition Dependent Traits in Birds and Fish
Chapter 4. Condition Dependent Traits in Arthropods and Mammals
Chapter 5. Sexual Selection and Human Vulnerability
Chapter 6. Human Vulnerability for Physical and Behavioral Traits
Chapter 7. Human Vulnerability for Brain and Cognitive Traits
Chapter 8. Implications for Human Health and Development
Evolution of vulnerability begins by reviewing the expansive literature on traits predicted to show sex-specific sensitivity to environmental and social stressors. It also details the implications for better assessing and understanding the consequences of exposure to these stressors. Next, the book reviews sexual selection - mate competition and choice - and the mechanisms involved in the evolution of condition-dependent traits and the stressors that can undermine their development and expression. These include poor early nutrition and health, parasites, social stress, and exposure to man-made toxins. Next, it reviews condition-dependent traits (physical, behavioral, cognitive, and brain) in birds, fish, insects, and mammals to demonstrate the ubiquity of these traits in nature. The focus then turns to humans and covers sex-specific vulnerabilities in children and adults regarding physical traits, social behavior, psychological wellbeing, and brain and cognitive traits. The sensitivity of these traits is related to the exposure to parasites, poor nutrition, social maltreatment, environmental toxins, chemotherapy, and Alzheimer's disease, among others. The book concludes with a chapter on implications that outlines how to better assess vulnerabilities in children and adults, and how to more fully understand how, why, and when in development certain types of environmental and social stressors are particularly harmful to humans.
The Value Added by an Evolutionary Perspective 3
Nonhuman Vulnerabilities 4
Human Vulnerabilities 7
The question of whether one sex or the other is more vulnerable to stressors is an intriguing and important one. Historically, the question has focused on the issue of male vulnerability (e.g., Greulich, 1951; Stini, 1969; Stinson, 1985). Even Darwin (1871) noted the excess of premature male mortality in many species, including the higher mortality of boys than girls during infancy. It is indeed the case that boys are more likely to die in infancy than girls, even with the dramatic declines in overall mortality over the past two centuries (Martin, 1949; Read, Troendle, & Klebanoff, 1997), and surviving boys are overrepresented among children with mild to serious medical or physical conditions (Jacobziner, Rich, Bleiberg, & Merchant, 1963). It is also the case that young men die at higher rates than young women - often as a direct result of male-on-male aggression (Wilson & Daly, 1985) or due to status seeking "showing off" (e.g., reckless driving; Evans, 2006) - and that men have a shorter life span than women (Allman, Rosin, Kumar, & Hasenstaub, 1998). These are certainly important vulnerabilities and can be placed in the context of the evolution of life histories (e.g., environmental influences on the timing of reproductive competition), some of which are discussed in Nesse and Williams's (1996) introduction to evolutionary medicine (see also Belsky, Steinberg, & Draper, 1991; Ellis, 2004; Figueredo et al., 2006).
However, they are not my focus. Rather, I am interested in the more nuanced questions of why some traits - specific physical features, behaviors, or cognitive competencies - are more easily disrupted by exposure to stressors than others, and why these trait-specific vulnerabilities can differ between the sexes and across species. For instance, why does poor nutrition during adolescence affect the height and physical fitness of boys more than girls (Prista, Maia, Damasceno, & Beunen, 2003), but the early stage of Alzheimer's disease affects the language competencies of women more than men (Henderson, Watt, & Galen Buckwalter, 1996)? In broader perspective, why does prenatal exposure to toxins compromise the spatial-navigation abilities of male deer mice (Peromyscus maniculatus), but leave unaffected the spatial abilities of same-species females or males of their cousin species, the California mouse (Peromyscus californicus; Jasarevic et al., 2011; Williams et al., 2013). Vulnerability from a life history perspective, in contrast, is focused on how exposure to stressors influences the timing (not disruption) of reproductive traits, such as age of menarche, or modifies how sexual relationships are formed and maintained (Del Giudice, 2009; Ellis & Del Giudice, 2014). Again, these are important issues, but beyond the scope of what I wish to accomplish in this book.
My goal is to outline and provide evidence for a simple conceptual model - traits that have been elaborated through sexual or social selection are especially vulnerable to disruption by exposure to environmental and social stressors - that allows us to understand the vulnerabilities of adolescent boys, women with Alzheimer's disease, and male deer mice, among many others, and places all of them in a unifying evolutionary context. The model enables the identification of sex- and species-specific traits whose development and expression are vulnerable to disruption by disease, poor nutrition, social stressors, and exposure to man-made toxins (e.g., environmental toxins and chemotherapy).
The concept that pulls cross-species vulnerabilities together is found with Darwin's (1871) sexual selection - competition for mates and mate choices - and West-Eberhard's (1983) social selection - competition for reproductively relevant resources (e.g., high-quality food) other than mates. The key is that these social dynamics result in the evolutionary exaggeration of traits that facilitate competition or that make one attractive to mates. These traits are either signaled directly (e.g., through physical size) or indirectly (e.g., through plumage coloration that is correlated with diet quality) and can be physical, behavioral, or involve brain and cognition, as will be illustrated in subsequent chapters. Whatever the trait, they are effective signals because they convey information about the individual's level of exposure to stressors and the ability to cope with them.
Identifying these traits and the conditions that can disrupt their expression is complicated, however, because a trait that signals competitive ability, for instance, in one sex or species may or may not signal competitive ability in the other sex or in other, even closely related species (Andersson, 1994). For either sex or any species, the identification of vulnerable traits requires an understanding of the evolutionary history of the species, in particular the traits that facilitate competition for mates and other resources and that influence mate choices. I provide the background needed to understand competition and choice and the sensitivity of the associated traits to environmental and social stressors in Chapter 2 and illustrate the ubiquity and diversity of these traits in Chapters 3 and 4.
I then apply these same principles to humans and detail the traits that I predict will be more vulnerable to stressors in boys and men, and the traits that I predict will be more vulnerable in girls and women. The existing literature on human sex differences does not allow for an evaluation of all of these predictions, but I provide proof of concept illustrations of sex differences in physical and behavioral vulnerabilities in Chapter 6 and in brain and cognitive vulnerabilities in Chapter 7. Implications for understanding and studying the nuances of human vulnerabilities are discussed in Chapter 8. I outline some of the key points of subsequent chapters in the second section below. In the first, I provide a few thoughts on why an evolutionary perspective on human vulnerabilities is important.
The Value Added by an Evolutionary Perspective
There are many things in the world that can be harmful to people, including premature birth, pre- and postnatal exposure to toxins, poor nutrition, infestation with parasites, poverty, and childhood maltreatment, among others. Indeed, these risks are well recognized and in many cases extensively studied (e.g., Hotez et al., 2008; Kim & Cicchetti, 2003), but they have not been framed in terms of sex differences in risk. The key to fully understanding the consequences of exposure to these potential hazards is to understand the traits that are most likely to be affected by them, and when in development these traits are most likely to be disrupted. Without this knowledge, we may assess traits that are not strongly affected by risk exposure, miss those that are affected, or assess the right traits but at...