The following appeared as part of a letter to the editor of a scientific journal.
“A recent study of eighteen rhesus monkeys provides clues as to the effects of birth order on an individual’s levels of stimulation. The study showed that in stimulating situations (such as an encounter with an unfamiliar monkey), firstborn infant monkeys produce up to twice as much of the hormone cortisol, which primes the body for increased activity levels, as do their younger siblings. Firstborn humans also produce relatively high levels of cortisol in stimulating situations (such as the return of a parent after an absence). The study also found that during pregnancy, first-time mother monkeys had higher levels of cortisol than did those who had had several offspring.”
Write a response in which you discuss one or more alternative explanations that could rival the proposed explanation and explain how your explanation(s) can plausibly account for the facts presented in the argument.
The author of the cited study may have inadvertently overlooked several relevant factors when making his/her conclusions, some of which are outlined below.
First, it is impossible to draw any definitive conclusions about the effects of birth order on humans, let alone non-human primates, by merely observing their behavior under laboratory conditions. The cited study was simply a measurement of hormone levels. The monkeys were not subjected to any stress, and the researcher did not manipulate their environment in any way. Thus, any increase in cortisol levels seen in the monkeys could be due to any number of factors, such as their birth order, the strain of the monkeys’ enclosure, the hormones or medicine they were taking, or their emotional state. In fact, the researcher himself admits that his/her study could not isolate these factors, and as such, any conclusions drawn about the effects of birth order are purely speculative and hypothetical.
Second, the monkeys did not experience a stimulating social situation, meaning that their reactions could not be equated with human reactions to such situations. While orangutans, chimpanzees, and bonobos all live in the social group, the monkeys in the study did not. Most likely, the study monkeys were exposed to their environment only through observation, which may or may not have been stressful. Furthermore, the fact that the firstborn monkeys in the experiment were significantly more active than the younger monkeys, as the researchers claim, does not necessarily indicate that they were more stimulated by their environment. It is possible that the firstborn monkeys simply had more energy because they were older, and therefore were more active simply by virtue of their greater body size. The fact that the firstborn monkeys produced more cortisol, however, is noteworthy, as cortisol plays a role in regulating energy metabolism. Thus, it is possible that increased levels of cortisol may explain the greater activity of the young monkeys, but not the older monkeys.
Third, the researcher’s claim that the differences in hormone levels between firstborn and younger siblings were statistically significant contradicts his/her own admission that the study was inconclusive. It is also unclear how the researcher was able to conclude that firstborn monkeys produced more cortisol. Since the hormone levels were measured in tiny samples taken from the monkeys, there is no way to tell whether the levels were higher or lower than the levels produced by monkeys of the same age. Even if the older and younger monkeys did differ in cortisol levels, the statistical difference in their levels may be explained by the fact that the animals were only measured once, rather than multiple times. Thus, the researcher’s statement that the ‘study found that firstborn monkeys produce up to twice as much of the hormone’ may not be entirely accurate.
Fourth, the researcher’s conclusion that firstborn monkeys and humans produce similar levels of cortisol in stimulating environments is based solely on the finding that firstborn monkeys produce higher levels of cortisol than younger siblings. This conclusion, however, is not adequately supported by the study, as firstborn monkeys produced similar levels of cortisol to younger siblings during stressful and non-stressful situations. Thus, there is little evidence that the monkeys’ increased levels of cortisol were specifically induced by a stimulating situation, and even less evidence that the levels were positively correlated with birth order.
Finally, the fact that the monkeys’ cortisol levels increased during pregnancy and that first-time mothers had higher cortisol levels than mothers who had had more children is inconsequential to the study’s conclusions. The monkeys’ hormone levels did not increase during pregnancy, and it was not proven that they were produced by the mothers. The monkeys’ levels of cortisol could have been higher simply by virtue of the fact that they were older and therefore were more stressed. Furthermore, the monkeys were tested only once, so any statistical differences in cortisol levels between the first-time mothers and the mothers of more children could have been caused by age rather than number of children. It is far more accurate to state that first-time mothers had higher levels of cortisol during pregnancy, than to state that firstborn monkeys produce more cortisol than younger siblings.