ARG-157

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.


In this letter to a science journal, it is concluded that it is the birth order that controls the levels of stimulation of a primate individual. The author presents a recent study in which researchers observed that in a stimulating environment, first-born infant monkeys produce up to twice as much of the hormone cortisol than their siblings. Cortisol, on the other hand, is known for regulating body’s level of activity and is regarded as an indicator of one’s stimulation level. Additionally, the author cites a study on human infants that show similar results. Therefore, he or she reaches the conclusion that the effects of birth order are well-established. While it may be true that the sequence of birth determines how responsive one can be to external stimuli, there are at least two competing hypotheses that could also account for the observations. I shall elaborate them below.


Firstly, as the author notes in his article but fails to explain in his theory, first-time mother monkeys had higher levels of cortisol than did those who had had several offspring. This leads to a new possibility in which the infants’ levels of stimulation are determined by the amount of cortisol available to them when they were still in the mother’s womb. In this way, because the first-time mother had the highest level of cortisol, this peak will be passed on to her offspring and exhibit itself as the levels of stimulation of that baby. The possible underlying mechanism of this theory is that during pregnancy the fetus receives all nutrients and hormones from his or her mother. As a result, it could be that exchange of substance that determines later physiological development after birth. Suppose that, in a different primate species, first-time mothers produce lower levels of cortisol, the first-borne infant is expected to show lower levels of activity in response to stimulation, if the hypothesis mentioned above is correct.


Furthermore, another hypothesis could be related to age and maturity. At the time of experiment, first-borne infants are apparently older than their younger siblings. This age differential could be key to the observed differences in the level of cortisol each individual infant produced. In other words, the older the infant becomes, the more responsive they are to a stimulating environment, exhibited by the amount of cortisol they could produce. In theory there could be two explanations. First, the older infants are physiologically and developmentally more mature, so its body is more capable producing the cortisol in the face of stimulation. Secondly, as an infant grows, his or her ability of conceiving the stimuli is also improved; that is to say, what used to be ignored by the infant can now be perceived as a form of stimulation and therefore can provoke hormone response. In sum, the improvement in physiological and cognitive capabilities associated with the age of an individual may ultimately control the stimulation level of an individual.


To summarize, two alternate hypotheses have been proposed above to account for the observations of infant monkeys and humans when they were faced with a stimulating environment, in addition to the one endorsed by the author. However, due to a lack of further information at hand, we are unable to make a rational judgment on which one of the three explanations is the best. More studies are certainly needed to shed more light on what is responsible for an individual’s level of stimulation. (568 words)

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