"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 of the letter states that in stimulating conditions first born monkeys(rhesus) produce more of the hormone cortisol than their younger siblings. He also conveys that first-time mothers had higher levels of cortisol than those who have had previous deliveries. First born humans have shown to produce more cortisol in stimulating situations, he states.

In the recent past studies have more often focussed on associations rather than implications. Like the one that the author states - as event A takes place more often with an even B is often interpreted as A implies B. This may well be the case in some studies but it doesnt work everywhere.

There could be various possible explanations for the proposed theory. It is natural for the older siblings look after the younger ones. This can better explain their higher alertness levels and related hormonal productions. Cortisol results in more intensive actions which are required while protecting the younger ones from dangerous situations.

Another alternate explanation could be that its the age that plays a major role in cortisol production. As people age they might require more cortisol to produce the same amount of reaction that they once did when they were younger. Agieng has also been shown to be a major factor in many hormonal changes during ones life cycle.

Although this is not the case in the pregnancy case presented here but the addition of more complications to one explanation like adaptability and conditioning to one particular situation might lead to a more feasible and acceptible rationale.