Thursday, March 10, 2011

The Life History of Men

I just finished reading this fascinating and informative book on the evolutionary and life history of men. I wanted to note a couple of key insights from the book here as they relate to themes I have been thinking about lately (and plan to return to later).

Why, one might ask, should we have an interest in the evolutionary and life history of men? Good question. Let's start by travelling over to this site, which has the list of mortality rates for young adults in the US.

If you look over the mortality rates for young adults there is one striking and alarming statistic the jumps out- the increased risk of death for males. Males ages 15-24, for example, die at nearly 3 times the rate of females in the same age cohort. For the 3 leading causes of death of young adults in this age category- accidents, homicides and suicides- men are 3 times more likely to die in an accident than women, 6 times more likely to be killed by homicide, and 5 times more likely to commit suicide. "Being male is now the single largest demographic risk factor for early mortality in developed countries" (source). Why is this so? [long aside: another important question is: why do societies seem much more content to tolerate much higher mortality rates for young men than they would for, say, women or children? No doubt part of the explanation is our socialization (as captured by the maxim "women and children first!" that applies to saving people from a sinking ship), but I also suspect a lot has to do with the evolution of our moral sensibilities. Sensibilities that evolved in environments were young men were viewed (due to necessity) as expendable. Still longer aside: bringing to the fore the disadvantages of being a young male will also offend many who prefer to perceive reality through the simplistic lens of the dichotomy of "the oppressors" vs "the oppressed". Men as a group tend to be lumped into the former, which thus makes it difficult for anyone to even raise the kinds of points I do here. I have faced this problem before, in my work on tackling aging. So I am well aware that I am starting from a counterintuitive position, but I ask for my dear reader's patience in hearing the case in full before dismissing it.]

Why are young men so vulnerable to accidents, homicide and suicide? Enter the "nature vs nurture" debate. No doubt nurture is important. But it is very unlikely that nurture alone could come close to explaining the higher mortality rates of young men. What kind of socialization are males exposed to that explains why they are 6 times more likely to die in homicide than are young females? Or 5 times more likely to take their own lives?

This disparity in mortality rates for males and females does not just occur in late adolescence/young adulthood. Males have a higher mortality rate at young ages (e.g. ages 1-4 the death rate for males is 12% higher than it is for females the same age) and older ages (e.g. ages 65-74 the death rate is 33% higher than it is for females that age). Considering the inequality in mortality rates between the genders across the lifespan makes it clear that it is not "nurture" alone that explains why males are more likely to die in every single age category, from the first year of life to age 85+.

This disparity between male and female mortality is likely to be missed if one just starts with the overall mortality rates for males and females in the US. In the year 2005 2,426,264 people died in the US (a death rate of 810.4 per 100 000). Of that number 1,201,942 were males, for a death rate of 814.8. This is only slightly higher than the death rate for females, which was 806.1. The total number of female deaths in that year was 1,224,322. This figure would suggest there is no significant mortality disparity between males and females. But once one considers the average age of death for the different genders the disparity becomes evident. 37.7% of female deaths were among those age 85 or older, whereas for the male deaths only 20% where in that age category. 65% of the deaths among females occurred in persons aged 75 or older. For men, only 46% of the deaths that year were males age 75 or older. The majority of male deaths occurred before age 75. This is a very significant disparity. And yet you will not likely hear any news report about this disparity, nor likely come across an academic paper in ethics or political philosophy addressing this inequality (this paper does, but the goal there is to refute egalitarianism rather than proposing to seriously tackle this inequality), nor find a politician championing the redressing of this inequality. Why? Food for thought for another day.

Understanding the physiological and evolutionary causes of the physical and behavioural differences of men and women could help us better address a whole host of important societal problems. And that is what motivated my interest in reading this book (along with the fact that I am the parent of three young males, so any help I can get in making sense of the brain of young males is much appreciated!). So let me now turn to the book.

I won't summarize the book's contents, but want to simply note a few important points I highlighted as I read through it, primarily for my own reference (and hopefully they will interest you as well).

The book starts with some obvious differences between men and women. On average, men are taller and physically stronger than women, we can't have babies, and we are involved in more than half of destructive and violent behaviour. We also age differently than women, and die sooner and faster (which explains the differences in mortality in late life between the sexes).

The focus of the book is on the evolutionary and life history of men. Evolutionary theory "explains the origins and development of species through time, while life history theory provides an explanation of the evolution of important life events such as growth and reproduction in a species" (2).

In the introduction there is a picture of a Aché (from Paraguay) male wearing a baseball cap that reads: "There are three stages to a man's life: Stud, Dud, and Thud". This nicely summarizes the life history of males (for a variety of species).

Males first emerged, along with sexual reproduction, about 2.5 billion years ago. Thus the male sex had been long established before our primate ancestors came into existence. And the male human body and brain we have today was influenced by the origins and evolution of the male sex.

Natural selection favours the development of a body and brain that maximizes lifetime reproductive success. Males face distinct challenges in this respect. Females must invest a lot more time and energy, relative to males, into reproduction in order to pass on their genes. After she becomes pregnant, an expecting mother's body undergoes drastic changes, as her body diverts food and nutrients to her potential offspring. But the energy needs, and strategies for managing energy, for men are different. Men do not menstruate, breast feed, etc. Investing more energy into body size and muscle development can increase reproductive success for males given the competition they face for mates (e.g. fending off rival competitors and also attracting mates). "...unlike mothers, fathers are not required by their biology to provide child support. Every calorie ingested by a human male is his to keep- and to invest, if he sees fit, in pursuits other than protecting and provisioning the younger generation" (221).

An important theme stressed throughout the book is the importance of "sex selection". "While natural selection is often the result of competition between individuals, sexual selection involves competition within each sex, competition to attract or get access to mates" (21). An important component of the life history of males is the trade-off between the energy needed to survive and the energy needed to secure opportunities to mate. My own thoughts: The reckless behaviour of young adult males today (which leads to the high mortality rates noted above) perhaps reflects (to a large extent) the tradeoff the male brain has undertaken in its development when circumstances were such that the males that invested more in reproductive effort were more successful at passing on their genes than those (more risk adverse) males that invested more in survivorship.

Testosterone emerges in the book as the main factor that explains the shorter life span, and more risky behaviour, of males. "Testosterone levels do change with age, with physical ramifications such as declines in muscle mass, greater fat deposition, especially around the midsection, possible changes in sexual function and motivation, as well as potential changes in psychological wellbeing" (168).

One of the driving forces that has shaped the bodies and minds of males is testosterone. This is a touchy subject, and the science is still incomplete. But here is a long passage that captures the author's nuanced stance on the issue:

The question of what happens when testosterone interacts with the brain during development has been a controversial topic, with its implication that behavioural differences between boys and girls or between men and women are to some extent influenced by biological factors before birth. Differences in behavior between boys and girls have often been attributed to cultural or social influences that mold sex roles. Certainly social factors have a strong influence, but more detailed analyses have shown that societal influences are but one aspect underlying sex-based differences. It would be naive to assume that millions of years of mammalian evolution involving differential selection pressures on males and females would result in a single brain type that is not selected to deal with sex-specific challenges. However, determining the selection pressures involved in male brain evolution and identifying the targets of selection are not easy tasks. It would not be unreasonable to suggest that the evolution of the male brain has been subject to selection pressures that address the optimal trade-off between neurological aspects related to survivorship and those related to reproductive effort. Would this involve selection for a tendency to exhibit sex-specific behaviours? It probably would. (83)

Important points to note about testosterone and men: there is a wide variation in T levels in men; T levels in American men are twice as high than in other populations around the world; lifestyle can influence T levels; testosterone drives the accelerated (but delayed) growth spurt of boys in adolescence.

On the chapter on "Sex and Fatherhood" the role of parental uncertainty is emphasized. Unlike female fertility, which is limited by the amount of energy involved in having offspring, male fertility is limited by the number of available mates. Furthermore, the evolution of internal fertilization also impacts the evolution of male behavior. Men, unlike women, can't be sure they are the father of the offspring. This leads to the evolution of mate-guarding strategies. The author notes that studies of American men suggest that contemporary American fathers invest more in genetically related children than in stepchildren. "Residence seems to be an important factor in male parental investment. It may be that, in human evolution, shared residence has suggested a greater chance of paternity" (148).

Other intriguing findings I wanted to note from the book:

- The hunting activities of our distant ancestors probably had an important impact on the development of the human brain, as food sources like meat made our larger brain possible.
-importance of Darwinian medicine is emphasized
-impact of CR and aging also addressed
"It is very likely that reproductive effort, extrinsic mortality, and sex-specific challenges to energy allocation strategies have played major roles in the evolution of human senescence" (201)
-interesting discussion of "male menopause"

The book was a fascinating read and has given me plenty to think about. It was reaffirmed my hunch that a better understanding of our evolutionary history is crucial if we hope to create a more fair and humane world. We are more likely to realize a future with less violence, less premature death, and less patriarchy and more love if we understand the role the evolutionary and life history of males has played in shaping our bodies and brains.

When time permits I will start reading this book, and hope to post some thoughts on that as well.