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 argues that there is a positive correlation between birth order and levels of stimulation. According to the study, when exposed to “unfamiliar” monkeys, the oldest siblings (i.e. first born) experience higher levels of stimulation than younger siblings. Though the methodology is not certain, it is clear that levels of stimulation are measured in the monkey’s levels of the hormone cortisol. The evidence supports the author’s argument, but in a weak and indirect manner.

It is clear that there is not enough evidence to definitively support the hypothesis. A sample size of 18 is not representative of the entire population. It would be negligent to assume this small study relates to other monkeys, and even worse to assume it applies to humans. Furthermore, there is no mention if the cortisol levels of human younger siblings were measured. Without this crucial evidence, there is no concrete evidence to support the correlation between the dubious monkey study and humans. However, there is stronger evidence to support an alternative theory.

An alternative explanation is the “novelty” theory.” Levels of stimulation, as measured by cortisol levels, are affected by the novelty of the experience rather than birth order. The study confirms that first born monkeys experience higher levels of cortisol when exposed to “unfamiliar” monkeys. However, the author neglects the intermediate variable, the degree of novelty. For the first born, interacting with someone other than his or her mother is a relatively novel experience. In a novel experience, the monkeys produce higher levels of cortisol. Conversely, younger siblings may be relatively acclimated to interacting with many other monkeys (i.e. older siblings), thus they produce less cortisol. Here, the term “unfamiliar” is uncertain, thus difficult to interpret. However, more compelling evidence lies at the end of the passage.

The final sentence of the argument provides additional support for the “novelty” theory. New mothers produce higher levels of cortisol than those who had several offspring. The first pregnancy is a novel experience for the mother, therefore she produces higher levels of cortisol than experienced mothers. Unless all the new mothers studied were first borns and all experienced mothers studied were younger siblings, which is not mentioned in the argument, the evidence supports the “novelty” theory better than the “birth order” theory.

In conclusion, the Rhetus Monkey study is best understood under the “novelty” theory, and not the “birth order” theory. By focusing solely on first born monkeys and humans, the author neglects the other important finding. New mothers’ first pregnancies are novel, therefore new mothers produce more cortisol. Applying the same theory to newborns, it is possible that firstborns find interacting with “unfamiliar” monkeys more novel because they lack older siblings. Taken as a whole, the alternative “novelty” theory is a more holistic explanation for the findings of the Rhetus Monkey study.