"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

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In the above letter, the author is trying to prove that birth order is the reason for the levels of stimulation in monkey or human children. Although, the reasoning in this letter seems to compelling, there are many defects which make the author’s explanation become infirm. Instead, there might be several other ways to elucidate the phenomenon stated in the paragraph.

Firstly, the firstborn infant monkeys might show higher level of cortison due to more severe situation they had to face not to the order they were born. More specifically, the unfamiliar monkey was presented in front of firstborn child is not the same as the one who was introduced with younger ones. The former might be more peevish and bothersome than the latter. In addition, while firstborn youngsters might encounter the alien monkeys alone, the younger ones possibly met the strange monkey with the appearance of other monkeys or human supervisors. Hence, the former situation was tenser than the latter.

Secondly, the higher concentration of cortison in firstborn monkeys can be attributed to previous stimulation rather than the order they were born. Cortison could be produced because of monkeys’ illness acquired prior to the experiment. More obviously, before conducting the trial, the firstborn monkeys can join a battle or a game with their pals. Those factors could seriously affect level of cortisol in their bodies.

Last, the discrepancy of cortisol level in experiments can be influenced by differential nature of individuals not by birth order. Particularly, this can be elucidated by the fact that the first child is more mature and has higher recognition level, while the younger ones are too small to perceive or discriminate surroundings. Similarly, when explaining the high level of first-time mother monkeys, it can be ascribed to physiological disparity of different individuals. In fact, author did not show the data of cortison concentration in the same female monkeys during their pregnancy of the first child compared with those data during their pregnancy of the youngers. Instead, he or she draw the conclusion based on comparison between first-time mother monkeys and other mother monkeys who had born more than one offspring. It is possibly because those first-time monkeys naturally have higher level of cortison than the counterparts.

In a nutshell, the explanation presented in the letter for higher level cortison in firstborn individuals including monkeys and humans is not reliable. This phenomenon might not because of birth order. On the contrary, some other possible answers can be made to better elucidate this phenomenon such as harsher condition when meeting strange monkeys, earlier stimulations and discrepancy in experimental objects.