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 argument describes wherein it was observed that in sample of rhesus monkeys, the first-born monkeys produced more hormone cortisol than their younger siblings. The argument also points out a similar observation in first born humans. Further another study shows that first-time mother monkeys have higher levels cortisol than those with several children. There are several loophooles in the argument which makes any conclusion deduced from it untenable.

Firstly, the study conducted on a sample of monkeys is unreliable due to two reasons. The primary reason being the small sample size used as part of the research. It is said the eighteen rhesus monkeys were examined in the study. Is this sample representative of all monkeys?. If not, than based on the results, no conlcusion can be made whether all first-born monkeys produce more cortisol in stimulating situations than younger ones. The second reason is that the study was conducted on one kind of monkeys - rhesus. Do all first-born monkeys other than rhesus kind also share similar traits?. It is completely possible that first-born monkeys of other kind do not share same behaviour. If such is the case, then to make any conclusion on basis of insufficent is unsound and baseless.

Further, it is said that first-time mother monkeys have higher levels of cortisol than other mothers with more than one offspring. What if this is the reason behind higher-level of cortisol in first-born monkeys and not because of any stimulating situations?. May be first-born monkeys are already having higher cortisol due to hereditary reasons. If this true, than despite any stimulating situations, first-born monkeys will always have higher cortisol than their younger siblings. Without ruling out other reasons of such occurences, it is erroneous to make any conclusion.

Also, the correlation between first-born human and monkeys do not support the argument. Are monkeys and human alike in all aspects?. Do the siblings of human have lower cortisol compared to their eldest?. If not, than the analogy between first-born monkeys and humans holds no ground. Without any evidence which shows that all first-born humans also produce higher cortiol levels than their siblings in any stimulating situtaion, the correlation does not justify the similar phenomenon in monkeys.

In sum, the argument is based on insubstantial evidences that highly undermines its validity. The study needs to do further research to find if there are other reasons behind this occurence in monkeys by using much a larger sample, assaying several other kinds of first-born monkeys.