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 writer of the letter assumes that the birth order effects the levels of stimulation in infants. Inside the letter the conclusion is based on three experiments. Citing the experiment on eighteen monkeys, the writer weakly supports the argument; experiment was based on a thriller reaction of infant monkeys encountered an unfamiliar monkey, which was seemingly more intense in firstborns than younger monkeys. The second experiment is that first born human children encounter one of their parents after an absent and seemingly they are more intense in responding to that stimuli. And in third experiment, mother monkeys in their first pregnancy show higher levels of cortisol than those mothers who had had several children. In what follows for every three experiments there are alternatives that not having been ruled out by the writer and the letter is thus unpersuasive.

The most competitive explanation for the experiment is that first born children_ as they are older_ might have a more developed sense of differentiating between familiar and unfamiliar faces in order to respond them. While it is not shown whether the tested monkeys were at a same age or not, the other explanation is equally possible that the results relate to the age of monkeys not their birth order. Furthermore, as it is not shown whether such children are having same sex, other explanation would be that the stimulation is conducted between different sexes and the firstborn who is for instance female is more sensitive to produce cortisol than the male one. Hence it can just be about their sex differences.

In the second experiment, other factors have a role in producing higher levels of cortisol not the hierarchy of birth. The cortisol level was relatively high, but relative to who? It is possible like the first experiment that relative to younger children the response is compared and the younger does not evolve enough to sense that stimuli, or as the children in lower years are more in touch with their mothers it is still possible that they recognize better their mother than their father; it is possible that facing father is compared to the other group faced their mothers and this was the reason why the stimulation were more intense in the latter group.

Finally the writer adduces another experiment in which during pregnancy, mother monkeys of first child produce higher levels of cortisol to support that why the hierarchy of being born has effect on the levels of stimulation. There are two alternative explanations that competes with the writer’s one. First, producing cortisol in higher measures can merely resulted because mothers lack any experience of pregnancy, and it is much more stressful than the following pregnancy. Hence it might not be related to the birth order. Furthermore, the cortisol in the mother womb might have not any effect after the infant is born.

In short, regarding all the rival explanations in the body paragraphs, there might be no clue about the effect of children’s order of birth on measures of stimulation.