Argument: 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

The author of the letter introduction a investigate result which perhaps reveal impact of birth order on an individual's levels of stimulation. To support the claim, the author cites three examples about the firstborn infant,firstborn humans and first-time mother monkey sequentially to prove that a positive relationship may be exists with birth order and the level of the hormone cortiso. Close scrutiny of each of the evidence, however, none of them is credible.

First at all, as for the first example, producing twice as much of the hormone cortisol doesn't indicate that there is a relation with birth order and the level of hormone,perhaps it may be caused by other factors such as gender:as I know,monkeys of different gender will have different level of hormone even if they share the same circumstance. According to it,there is a flaw in the research that it doesn't remind gender and so that they can't rule out a following situation that all the first infant monkeys in experiment are female who will actually secrete more hormones. Even if they are not first infant,they can also produce the highest level of hormones. In short, the factors that have impact on the level of stimulation may be monkey's gender rather than birth order.

Second, about the firstborn humans, the experiment is also not credible.It's entirely possible that the firstborn infant doesn't have playmates at home, so parents are their only friends. After a period time doesn't see their parents, they will be so excited and then produce more hormones when parents come home. On the contrast, as for the later born infant, they use to play with their sisters or brothers,so maybe they don't mind whether their parents are at home or not.

Third, about the last experiment, the author assume unfairly that first-time mother monkeys who have higher levels of hormone will delivery an infant monkey that also have relatively more hormone. However, the author use no evidence to support the assumption.Lacking such evidence it is equally possible that the hormone level of mothers don't influence the offspring's. Thus, the author 's claim is not credible.

In sum, all these three examples can't attest the relationship with birth order and the level of hormone of cortiso.The results from the experiments may be influent by other factors,such as gender. So to strength the claim, the author should rule out all other possible factors.