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 argumet presents some observations to infer that first born monkeys usually have higher level of stimulation hormones than the next siblings. Even though it seems that cause and effect are well related in this argument, some points are missing and probably other explanations could rationalize the observations.

First obeservation is that when older infants see and unfamiliar monkey they tend to secrete more cortisol. Seeing an unfamiliar monkey might have many meanings. For example, if all of the siblings are facing this unfamiliar monkey, it is natural that older siblings incline to protect the younger ones. Perhaps, this could be one reason why first born monkeys produce more cortisol in such situations. In another, it is possible that the unfamiliar monkey is a prospective mate for the first-born and this simply causes the first-born to produce more hormone. Since exprementing on only 18 monkeys is not much, and also there is no information about the attributes of the unfamilair monkey and situations, the conclusion is not persuasive and the mentioned alternatives are all likely to be the reason.

The second inference about generating more hormone when seeing a parent by older sibling is not spported well. It is clear that usually all the siblings get excited to the same extent while visting their parents which has been absent for long. Hence, maybe this difference in cortisol level is due to difference in age. Particulary, it is possible that until some age the amount of this hormone increase as the monkey ages. As a result, younger siblings do not generate more hormone because the proportion is still low in their bodies.

Finally, the discussion about the first time mothers might be flawed. A reasonable explanation could be the circumstances the mother monkey is experiencing during pregnancy. For example, if we examine the hormone levels in a mother monkey that is giving birth two its third infant while the previous two infants have died during pregnancy, it is possible that its hormone levels are even higher than a first time mother. More specifically, the amount of fear or unfamiliraity with the situation could be responsible for secreting more hormone during pregnancy.

All in all, in order to draw more accurate conclusions it is need to have more information about other aspects of all the monkey participating in the experiment. Obviously, many other factors including situtaion of the experiment, mental history of monkeys and age could alter the results drasically. Thus, these evidens are not likely to lead us to a unique explanation for the observations.