"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

The argument in which that the birth order have influence in the amount of the hormone cortisol and the range of activity seems tenable at first glance. The author makes a valid argument, one that it would be correct, if his premises were true. While, he provides the assumption for which there is no clear evidence and it uses the terms that lack the definition.

First, the author claims the firstborn mankind produces the hormone cortisol in stimulating situations by the study on the eighteen monkeys. Making judgment on man just by one investigation of the monkeys is skeptical and defect. The author doesn't provide more cases and points for convincing, so that it seems impartial and incomplete. While citing the some clues, there are no more evidences about it. Maybe, the stimulating situations mentioned about the monkeys are different from the men. Likewise, the cortisol hormone might act special on eighteen monkey rather than other monkeys and human.

Second, the author implies the firstborn people produce the higher level of hormone cortisol rather than other siblings. While, there aren't provided more explanation about how the hormone cortisol is measures in monkeys, and how it acts to increase the level of the activity. The author doesn't consider how the comparison had been done among the monkeys and what other simulating situations might be considered on the human. It is still doubtful on such conclusion because it can be quite converse in the other situations.

Finally, according the argument, first-time mother monkeys have the high level of the hormone cortisol. While, the author doesn't make a clear correlation between the other points stated before. What is the influence of the first-time monkeys on the firstborn individuals and how it is done completely? How can it possible to make a total conclusion about the human through the experiments on the monkeys? To answer these questions, the author needs more scientific reasoning.

As a result, the argument appears to rely on the obscure experiment on the monkeys, while it lacks more conclusive supported points on the details. The author can easily address these issues by providing an introduction and statement on the experiment, situations, and the physical and mental differences or correlations between of the human and monkey in various situations.