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 situ

The facts presented in the argument include: "in stimulating situations, firstborn infant monkeys produce up to twice as much of the hormone cortisol" "as do their younger siblings" and "Firstborn humans also produce relatively high levels of cortisol in stimulating situations". From the above facts, the author has proposed that the difference in levels of production of cortisol, and thus the anticipated increase in activity in response to stimulants, was simply directly related to the order of their births. However, there are several explanations for the experimental results which can rival the proposed explanation.
Firstly, the author also reported a higher levels of cortisol in mother monkeys who have the first child than did those who had had several offspring; which implied that the mother monkeys might behave differently in taking care of their firstborn infant monkey and their other infant monkeys: the infants might feel the stress from their mothers and stress out as well.
In addition, we know that during pregnancy, mothers and embryos exchange their blood and other nutrition, so it is also possible that some maternal-derived factors acquired during pregnancy still affect the infant cortisol levels. In either cases, the difference in level of cortisol in infants results from mother's cortisol levels and other factors rather the order of birth.
Then, the different experience of the infant may also account for such difference in cortisol level produced upon stimulation. For example, the firstborn monkeys and humans are only surrounded by their parents while the later infants are surrounded by their siblings too. Surrounded by more people or animals, the non-firstborn infants might feel more secured than the firstborn infants. This factor is particular important when the stimulating test was encountering an unfamiliar monkey or the absence of parents.

In conclusion, there are several plausible explanations which can account for the facts presented in the arguments: the difference in level of experience of mothers, the presence of other maternal-derived blood factors that affect cortisol level, or the difference in experience of infants with outside world when they have or do not have siblings. Such alternative explanations should be ruled out for any conclusion to be reached.