"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 suggests that among rhesus monkeys and human beings, a person who is born earlier is more likely to experience higher levels of stimulation in terms of the release of cortisol, when exposed to stress inducing situations such as an encounter with a stranger or the return of a parent after long absence. The author further suggests that the birth order itself directly influences the production of cortisol.

However, this difference could be explained through differences in upbringing as opposed to birth order. The conditions in which parents bring up two different children vary wildly and previous parental experience could be a factor in determining a child's tendency to release cortisol. Prior experience could help a parent in understand how better to pacify an infant, thereby reducing the production of cortisol in the infant. Hence it need not be birth order in itself, but parental maturity and capability that influences cortisol production in children.

The author quotes a study in which first-time mothers had higher levels of cortisol than those who had several offspring. While it is quite possible for first-time mothers to be stressed, it is unlikely to be a biological imperative. The same study could be used to argue that ill-equipped or unprepared mothers were likely to feel overstimulated, leading to stress and anxiety, which reflected in an over-stimulation of the infant. It may be that the mother was preoccupied with regulating her own hormonal overwhelm, as opposed to soothing the infant. One would need to perform an experiment in which a first-born infant is taken care of by a well prepared and informed mother, to conclude that it is birth order or parental experience that matters as opposed to learned skills. Furthermore, a greater psychological insight into attachment theory might be needed to resolve the role of maternal influence on the development of a child.

Another flaw in the author’s argument is the unspoken assumption of correlation implying causation. There is the possibility of wild temperamental differences among offspring. It may be that in the small sample that the investigators chose, there were wild fluctuations in terms of predispositions. One sibling might have been born with a lower tendency to release cortisol as opposed to another and this might have no correlation whatsoever with their birth order. One way to resolve this would be through increasing the sample size. A stronger rebuttal of this explanation however would rely on a biochemical theory explaining the connection between cortisol and birth order.

Thus there are at least two alternative explanations to the one proposed by the author. A distinguishing evidence would have to rely a psychological study of the impact of parental experience on upbringing, a larger sample size or a biochemical understanding of the relation between cortisol and birth order.