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 passage claimed that, according to an experimental study, the birth order can influence the level of stimulation in rhesus monkeys. The research studied the blood level of cortisol, which is the primery hormone related to stress response in animals, and reported the positive relationship between cortisol increase and birth order.

At a quick glance, the conclusion seems to be some faulty. The first factor that undermines the validity of such result is the number of cases studied in that experiment. Although animal studies are not easy to perform on large population, but the limited number of monkeys in this study weakens the results. No one can firmly assert that in a group with for example tripled numbers of monkeys the results will remain constant.

The second thing that questioned the reliability of results is that first-birth monkeys were compared with their younger siblings in terms of cortisol levels. One of the prominent factors in all experimental studies is matching the study groups. As the mammal offspring are not fully developed at birth, most of their physiologic functions will gradually develop over a long period of time till adulthood. The author should declare that in what exact time the baby monkeys are capable of producing cortisol in a level comparable to adults. Otherwise, one can argue that the lower level of cortisol in younger cases might be due to underdeveloped adrenal gland function.

The author mentioned the high blood concentration of cortisol in first-mother monkeys as another finding to imply that because of higher excretion of cortisol by first child fetus, mother monkey experience increased cortisol level during first pregnancy. Refuting this suggestion, it is obvious that first pregnancy is an unknown and stressful experience for all female animals. This is nothing to do with the fetus cortisol excretion, especially while considering that the under developed embryo cannot produce that amount of cortisol before birth.

On the whole, the methodology of experimental studies should be meticulously adopted otherwise the results and conclusion will fail to support a claim.