Tuesday, February 19, 2008

Reproducing in Cities: recent paper by R. Mace

A recent perspective by Ruth Mace published in Science (subscription required) gives a perspective on the demographic transition - the transition to reduced fertility rates among wealthy nations and economic classes. The argument is based on the costs of childrearing that seem to increase with urbanization and the lowered rates of infant and child mortality that can accompany city life in contexts where sufficient health care and sanitation are available.

Here's the paper's abstract:

"Reproducing in cities has always been costly, leading to lower fertility (that is, lower birth rates) in urban than in rural areas. Historically, although cities provided job opportunities, initially residents incurred the penalty of higher infant mortality, but as mortality rates fell at the end of the 19th century, European birth rates began to plummet. Fertility decline in Africa only started recently and has been dramatic in some cities. Here it is argued that both historical and evolutionary demographers are interpreting fertility declines across the globe in terms of the relative costs of child rearing, which increase to allow children to out compete their peers. Now largely free from the fear of early death, postindustrial societies may create an environment that generates runaway parental investment, which will continue to drive fertility ever lower."

The paper provides an interesting perspective on fertility in urban life and of course on the demographic transition in general. Mace essentially argues that the demographic transition is the result of the quantity quality tradeoff.

"Industrialization and urban living enable new professions to emerge, some of which are only available to those invested with considerable capital or training." .... "Whether perceived relative costs are equivalent to actual costs is a moot point; but, in essence, historical and evolutionary demographers are converging on similar explanations for demographic change. The cost of raising a child includes enabling it to compete with its peers—for marriage partners, for jobs, or for the means to support a family—and if that competition increases costs, then basic evolutionary ecology predicts that optimal fertility will decline (16). Education introduced a new mechanism through which children could compete for future employment opportunities. School also adds pressure on parents to present adequately fed and dressed offspring for public scrutiny."

Figure 2, pasted in below, represents the basic schematic for the argument. Mace says that in many instances wealth and fertility are often positively correlated within these 'homogenous subpopulations'. While this may indeed be true (I think that it is and would argue in favor of that point), the data supporting it are really not that good and the source Mace cites as proving
that wealth and family size are usually positively correlated really provides only weak support and is based on little data.

Fig. 2. Schematic diagram of how different levels of parental investment per child can generate positive relationships between fertility and wealth in subpopulations (where each of the diagonals represents a different subpopulation within a larger society) but a negative relationship between wealth and fertility over a large heterogeneous population. Levels of parental investment per child may be highest in urban areas.

Mace is skeptical the opinion that these trends in fertility decline simply represent shifts in cultural values that have rapidly swept over the globe and I think for good reason. Clearly the pattern is consistent in very different cultural contexts (Europe, Americas, and Africa have all gone through similar demographic transitions following broadly similar correlations between economic diversification and urbanization). Instead, there is good evidence for a general evolutionary mechanism at work. So the general trend of having "few higher quality offspring" started as initially advantageous, accelerated with lower levels of child mortality and the increased importance of inherited wealth and status, and then became subject to runaway selection.

"Transfers of resources from parents to offspring are key to understanding human life-history evolution (23). In wealth-owning societies, siblings compete with each other for their parents' material and intellectual resources (2426). If parental investment is a key influence on children's future success, and the ability to invest effectively in children is heritable (and cultural traits such as wealth and status are usually highly heritable), then it is possible that runaway cultural selection has occurred in preferred levels of investment in each child (27), driving the quantity/quality trade-off further in the direction of offspring quality. Hence, I argue that the emergence of postindustrial life, now largely free from the fear of early mortality, seems to have generated conditions under which a runaway process of ever-escalating levels of investment in our children continues to drive fertility ever lower."

The link to 'cultural selection' could be a bit more clear in order to differentiate it from the widely spreading cultural shift idea of some cultural demographers which she criticizes earlier in the paper. Readers who are not already on Mace's side of the argument may not see the difference, and its not entirely clear. However, I am on Mace's side of the argument and think that evolutionary mechanisms do indeed underly fertility decisions of this sort. The runaway selection idea is one that deserves more attention. I'd like to see some modeling or analytical work showing exactly how it could lead to below replacement fertility. Perhaps some of the references in the paper accomplish this. On the whole, its good to see this sort of approach being applied in human demography. Being a UNM graduate student though, I couldn't help but notice that some of the highly relevant work done by some faculty and/0r grad students here was not cited or included in the paper (e.g., Moses and Brown 2003; Kaplan 1996), but that is clearly only a gripe driven by hometown bias.

Kaplan H. (1996) A Theory of Fertility and Parental Investment in Traditional and Modern Human Societies. Yearbook of Physical Anthropology, 39, 91 - 135

Moses M.E. & Brown J.H. (2003) Allometry of human fertility and energy use. Ecology Letters, 6, 295-300


Serge said...
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Serge said...

In addition to the link of mortality rates being affected by urbanization. Its also interesting that urbanizes living has more , crime, higher STDs and as stated low-birth-weight babies but they tend to have a higher standards for overall health.(perhaps due to better information) rural areas of American is shown to higher concentration of obesity and chronic health problems. The ways to resolve this issue is for society/ people to receives the benefits of city / urban living while focusing on the lifestyle of country living.

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