In the preceding argument the author claim a connection between the birth order and the individual monkey's or human's reaction on stimlation, citing the evidence that firstborn rhesus monkeys show a higher cortisol level upon stimulation. Although the argument seems appealing at first sight, upon careful scrutiny, it is rife with wholes and flaws
Firstly, the argument is based on a small samples of a single species. As the author mentioned, only eighteen rhesus monkeys were used for the experiment. But species varies greatly in their physiology, thus the result got from rhesus monkeys may not be a generalized one and cannot be applied to other species as a whole. Additionally, only eighteen monkeys were included, which is a relatively small sample volume, making the result not persuasive.
Secondly, the author of the letter use cortisol level as a measurement of the stimulus. Although cortisol may be an important hormone secreted upon stimulation, we have to bear it in mind that organisms are a complex system, and it is quite likely that other hormones or chemicals also play an indispensable role in the reaction to stimulus, and are not considered. Also, cortisol may have other rules other than stimulus correspondence. Thus, use corticol level to present the monkeys reaction to stimulus can be invalid. The author need to show more data from different perspective to support his or her argument.
Last but not least, given that the use of cortisol as a measurement is proper and valid, the real cause of the augmented cortisol level can be explained other reasons, such as parental influence. The author stated mother monkeys show more cortisol as well, so there's a the possibility that the mother monkeys release more cortisol for some unknown reasons, and these molecules diffuse into the baby's blood, giving a misleading scenario that baby monkeys release more cortisol by themselves. Further investigation is needed to make this point clear.
Summing up, although not completely wrong, the author's augument is based on many premises and assumptions that have not been tested scientifically. A thorough study is of vital importance to should be carried out to support or optimize the author's argument above.