"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 author of the argument asserts that the fact that order of birth impacts on individual's level of stimulation has been proven by a recent study. The conclusion is buttressed by several evidences which may have alternative explanations and thus, the conclusion may be unwarranted.

First of all we were told that a recent study with rhesus monkeys displays that the first born creature has higher level of cortisol. First of all, the study was conducted with a relatively small group of one kind of monkey, the probability exists that these findings are characteristics of this particular species of monkey. May we assert that the first born dog have higher level of cortisol? Perhaps the answer is "no" because we have no proof that two species react similarly on a stressful situation. In other words, the conclusion should be restricted otherwise it has unreasonably wide extrapolation. However, even if the level of the cortisol is higher for all first born mammals, perhaps some other factors may cause the increase of the level of the hormone. For instance, the first born monkey may be elder than other siblings and therefore be more developed than others, consequently, the first born monkey may have more developed organism which more swiftly reacts on danger. Additionally to it, rhesus monkeys are social animals and probably the first born animal is the head of the family’s offspring and therefore it should encounter with the enemy. The argument could have been stronger had the author demonstrated that if the first born monkey is taken from family the second born monkey does not undergo enhancement of a level of cortisol. In this case, we would have proven that the order of birth directly impacts on the individual's level of stimulation.

The second evidence which is given to us is that the first born humans also have high level of cortisol in stimulating situations. However, this may be an adequate reaction of a healthy organism on stimulation situation. Perhaps the second born child undergoes the same reaction if one will meet the parent who has returned from absence.

Finally, the argument claims that the first-time mother monkeys had higher level of cortisol than monkeys which have already had offspring. One of the possible explanations is that the improvement of level of cortisol is a natural reaction on the stressful situation. The monkey which has never undergone the process of birth may be instinctively anxious about its conditions.

In conclusion, perhaps cortisol is a hormone of stress in this case the enhancement of the level of the cortisol in the blood stream of the first born rhesus monkey may be explained by the fact that it is elder and therefore physiologically better developed than its brothers and sisters, moreover, it has a social role which assume that it should protect its sibling. Moreover, the reaction of first born human may be a natural reaction on emotions which are revoked by parent's appearance. Consequently, the influence of order of birth on the individual's level of stimulation is not proven.