In the letter, the author adduces recent studies of rhesus monkeys and observations in human to substantiate the point that birth order can affect an individual\'s levels of stimulation. While this point of view may sound plausible at first, a closer review brings about other probable interpretations of the experiment results, which definitely undermine the author's explanations.
To begin with, the author claims that firstborn infant monkeys generate twice as much of cortisol than their younger siblings facing stimuli. Nonetheless, the impact of age cannot be ruled out here. It is likely that the firstborn monkeys produce more hormone simply because they are older and more mature. If this were the case, the differences between amount of hormone cortisol produced would be a function of age, but has little to do with the birth order of monkeys. What is more, how the experiment was carried out remains elusive. May be levels of stimulation of firstborn monkeys were measured in the presence of their younger siblings, so the behavior is, in fact, affected only by other younger monkeys nearby, instead of being determined by the birth order.
Additionally, the observation of similar phenomena in human also needs to be viewed with caution. Although human and rhesus share similarities in many ways, they can be strikingly different. If the obversed behavior in human resultes from the language communication between parents and children, which the monkeys lack, then it would be too reckless for the author to see it as an effect of the birth order.
Last but not least, the author fails to articulate clearly the relationship between the levels of stimulation of offsprings and the higher cortisol concentration in blood of first-mothers. For instance, may be firstborn individuals produce more cortisol under stimulations simply because of the influence their mothers, who were first-time mother, and this effect can fade as they grow older. Therefore, the levels of stimulations may not be correlated to birth order at all, and what the author observed is only the short-term influence from first-mothers.
In sum, in the letter an experiment on rhesus monkeys and observations on human are briefly described, but the conclusion drew by the author is not supported by the evidence. There are many possible explanations which also fit well with the phenomena, and these alternative interpretations pose a great challenge to the author\'s theory unless more ditails about the experiment can be provided.