More is not Necessarily Better: Making Babies in the Modern World.

Offspring: Human Fertility Behavior in Biodemographic Perspective. Edited by Kenneth W. Wachter and Rudolpho A. Bulatao. Washington, DC: The National Academies Press, 2003, 379 pages. Softcover, $59.00.

Consilience, the conceptual unity of all knowledge (Wilson, 1998), is an idea that most scientists generally ignore because of its impracticality. Yes, it is theoretically possible to understand political science from the perspective of proteomics, for example, but no one currently has either the money or the time to attempt such a project. Yet we live in an era when occasionally one encounters a book whose authors, in the aggregate, come close to providing a coherent multidisciplinary explanation for a phenomenon. Offspring is such a book, and its topic is the myriad factors that influence human decisions to have children.

The emerging problem of declining fertility in Western European and some developing countries motivated the Panel on Population to convene a meeting and to produce this book. This is a problem because, in many cases, fertility rates are at subreplacement levels, leading to real declines in population. However, decreasing fertility in developing countries may be a sign that the quality of life is improving for people, particularly women. As indicated in several chapters, educational achievement is negatively correlated with fertility. As women become more educated, they have fewer children. But beyond the implications of declining fertility for public policy, fertility poses a fascinating puzzle for those of us intrigued by the concepts of evolutionary psychology. If our brains and bodies are merely vehicles with which genes reproduce themselves, why are increasing numbers of humans behaving in such an apparently maladaptive manner?

The contents of the book draw upon several disciplines, ranging from molecular biology to socioeconomic modeling. Together, they achieve the impressive feat of conveying a lot of information about the many facets of reproductive behavior while keeping the reader interested in finding an answer to the paradox. It is beyond the scope of this review to discuss each chapter individually, but I found them all to be of good quality and appropriate for inclusion within the book. While Offspring is not a textbook designed for pedagogy, several chapters provide the non-expert with excellent introductions to the field under review, such as behavioral genetics and behavioral neuroendocrinology.

For example, Judy Cameron's chapter should be read by those who need a succinct introduction to hormones and behavior. She gives a lucid review of the latest issues in developmental and behavioral neuroendocrinology, including discoveries about the sexual differentiation of the hypothalamic-pituitary-gonadal axis and an update on the interactions between stress and reproductive function. Ultimately, of course, the demographic patterns of human fertility behavior at the heart of this book are mediated by societies of individuals, each possessing a brain). Scientists who study phenomena at the group and population levels will appreciate Cameron's description of the proximal mechanisms by which all mammals realize their evolutionary mandate.

Larry Young provides a readable and fascinating description of his research into two closely related species of vole that seem to differ only in terms of their habitat and mating pattern. The male of one species is a reclusive "cad," hanging around only long enough to ejaculate, whereas the other is a perfect "dad"--utterly monogamous and a doting parent. Young and colleagues have found that the two vole species differ in the expression level of a certain receptor in the reward and reinforcement part of the brain. When the expression of the receptor was artificially increased using a gene therapy technique, affiliation toward pups and other conspecifics also increased. Young points to two other pairs of closely related species that likewise differ in their affiliative behavior. Each one of the pairs has been found to differ in the same neurochemical way as the voles.

Tempted as I was to believe that interindividual and cultural differences in "caddishness" and "daddishness" simply reflect a genetic difference as straightforward as that seen in voles, the chapters on behavioral genetics disabused me of that notion. The power of behavioral genetics is that it allows researchers to dissect the relative roles of nature and nurture for a given trait. Two chapters, one by Michael Rutter and the other by Hans Peter Kohler and Joseph Rodgers, provide clear evidence for a strong genetic component of human fertility. However, they also discuss important confounding factors that must be taken into account when interpreting such findings, such as the effect of the genome on the choices one makes about the environment one chooses to live in, and conversely, the effect of the environment on gene expression in the brain.

Perhaps the most striking finding to come from the behavioral genetics chapters is concerned with the interaction of increased educational opportunity with heredity in the expression of fertility behavior. As noted above, educational achievement of women is negatively correlated with fertility. Yet as educational opportunities increase within a society, so too does the heritability of fertility! That is, genes play a greater role in determining the number of children a woman may have in a society such as the U.S., where primary and secondary education are within reach of virtually every citizen, than in an impoverished or repressive society, where factors beyond the individual's control largely prevent her self-actualization.

Surprisingly, sexual selection receives little attention from most authors despite its potential to influence those genes involved in fertility. Sexual selection, of course, is the force that drives the evolution of traits designed solely to enhance reproductive success rather than survival. The power of sexual selection lies in its positive feedback, as males evolve better skills, tougher weapons, and more grandiose ornamentation in an environment of increasingly critical females. The evolution of each sex within the other sex's environment leads to the rapid divergence of male and female behavioral strategies and increases the variability of sexually selected traits within the population. Numerous authors in Offspring contemplate the evolutionary mechanisms by which fertility behavior can change, yet all but two, Young and Gangestad, fail to acknowledge that sexual selection may play a role.

Steve Gangestad's chapter on the antagonistic coevolution of males and females is simply fascinating. He describes clever experiments with fruit flies that show how artificial manipulations of the social environment can lead to dramatic changes in mating behavior and reproductive physiology. For example, male fruit flies include a mild toxin in their seminal fluid that harms other males' sperm and reduces the tendency of the female to re-mate. If monogamy is artificially enforced upon a colony of fruit flies, thus reducing intermale competition, seminal fluid toxicity is reduced and females' fertility is increased within 45 generations.

As this example illustrates, environmental constraints can shape the reproductive strategies employed by males and females--strategies which are continually evolving as each sex maximizes its reproductive success. In contrast to the seemingly hard-wired proclivities of Drosophila melanogaster, Gangestad suggests that humans may have evolved the ability to adapt our fertility behavior to environmental contingency. For example, improvements in environmental quality could multiply the effects of parental investment on the quality of offspring. An environment rich in resources may thus promote the expression of high parental investment coupled with relatively low fertility. Although it is speculative, like every hypothesis about trends in human fertility behavior, Gangestad articulates a theory to explain why people are having fewer babies. Simply put, conditions are such that greater investment in children now is expected to lead to increased numbers of grandchildren in the next generation. Whether this is so awaits proof, but it is the only way to provide an evolutionary rationale for the phenomenon. Or is it?

Perhaps the finest virtue of Offspring is that it reminded me that the "mental mechanisms" spoken of by evolutionary psychologists such as David Buss, Donald Symons, or John Tooby and Leda Cosmides accomplish their tasks in ways we often might not expect. Our feelings of lust, attachment, nurturance, jealousy, and love are all manifestations of evolution working its way into our minds. So what if the numbers of our descendants are declining below replacement levels? The function of these mechanisms is to love and foster children; it just so happens that in certain circumstances they have increased reproductive success, leading to their selection. But since we live in a culture where parents of means can spend thousands in fertility clinics just to have one child, or wrack their brains seeking the trendiest name for their baby, or ensure their offspring's well-being with the finest health care and education, is it any wonder that we would rather love a few a lot than to have many and neglect them (relatively speaking)?

In summary, Offspring is an excellent book. It presents 13 interesting yet sophisticated chapters about one subject, and each addresses human fertility in a different way. No one knows for sure why humans behave they way they do when it comes to making love and having babies, but this book will convince you that some very bright people are thinking a great deal about the subject.

REFERENCES

Wilson, E. O. (1998). Consilience: The unity of knowledge. New York: Vintage.

Reviewed by Bradley M. Cooke, Ph.D., WCAS Neurobiology & Physiology, HOGAN 2205 Tech Drive, #2-160, Evanston, IL 60208; e-mail: b-cooke @northwestern.edu.