Prenatal “Shocks” and Birth Outcomes

Birth weight is often used as a rough gauge for the quality of the prenatal environment. A newborn who weighs 2500g or less (about 5.5 pounds) is considered to be “low birth weight” (LBW). At the individual level, weight alone is an imperfect measure because of confounders such as gestational length (it’s axiomatic that the less time spent in the womb, the less time there is to grow). However, at the population level, if average birth weight fluctuates, then it is an indication that something in the environment probably has changed.

Sometimes, stressful changes can be low-intensity and chronic; at other times, they can be abrupt and dramatic. Biologists, psychologists, and bioanthropologists might call these changes “stressors” or “insults.” Economists might use the term “shocks.” They’re both getting at the same idea: to what extent can harmful environmental factors affect growth and health outcomes? 

In the case of a natural disaster, the harm done can be substantial. Florencia Torche (2011) found that rates of LBW increased following the 2005 Tarapaca earthquake in northern Chile. Despite the magnitude of the earthquake (7.9 on the Richter scale), the amount of destruction was relatively limited: eleven people died, and 0.035% of the population had to temporarily relocate to shelters. This was attributed to the low population density of the region as well as Chilean preparedness and building codes to withstand earthquakes. Although the damage was not as severe as it could have been, Torche reasoned that the earthquake likely caused acute maternal stress, which in turn could affect prenatal development. 

Looking at over half a million births, Torche used maternal county of residence as an estimate of the earthquake’s intensity across different trimesters of exposure. She found that mothers who were lived in the most intensely affected regions during the first trimester were the most affected. The probability of LBW increased from 4.7% to 6.5%, while rates of pre-term births also increased from 5.2% to 8.0%. Later periods of gestation were not substantially affected, and for infants who were conceived after the earthquake, the probability of LBW returned to baseline.

Again, these outcomes seemed to result primarily from acute psychological stress stemming from the earthquake. Torche reasoned that – given the relatively low amount of damage to infrastructure – the increases in LBW and pre-term births were unlikely to have resulted from other factors such as malnutrition, infection, stress resulting from deprivation, strenuous workloads, or exposure to environmental toxins. In reality, it’s not possible to control for all of these variables entirely, but overall it seems plausible that maternal psychological stress played a substantial role in birth outcomes.

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Darwin, Oversimplified

ChuckDCharles Darwin recognized early on the potential for people to oversimplify his writing. In particular, he was adamant that not everything in evolution was the result of adaptation by natural selection, though he worried that others would caricature his work in that way. In other words, Darwin was not as Darwinian as he is sometimes portrayed. The following passage comes from the sixth edition of On the Origin of Species (1872: 421):



“I have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified, during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner, that is in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection. But as my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous position—namely, at the close of the Introduction—the following words: “I am convinced that natural selection has been the main but not the exclusive means of modification.” This has been of no avail. Great is the power of steady misrepresentation; but the history of science shows that fortunately this power does not long endure.”

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