Rhesus Monkeys

The author observed high levels of cortisol in firstborn infant monkeys in stimulating environment compared with that of its siblings. The author attributes this phenomenon to firstborns showing greater level of stimulation. However, this result could easily explained by other means.
Assuming that the experimental data is significant, there could be many alternatives to explaining this situation that could not be eliminated before more evidence is provided.
The author does not provide details of the experiment. Do the subjects of the test come from the same mother? Are the subject tested together at the same time? It could just be that firstborns are older during the time of the test, and thus have a greater cognitive understanding of the outside stimuli. Even if the experiments are conducted separately, there could be other explanations.
For example, the author does not specify whether the high level of cortisol released is actually synthesized by the monkey due to stress, or as a side effect from the mother. It could simply be that high stress level experienced by first time mothers produced excess cortisol that passed on to its infant. The infant, used to high levels of cortisol, receptors sensitive to cortisol might be damaged, thus the infant would continue to synthesize cortisol at high levels at all times while exhibiting the same level of response to stress. Another scenario would be that, being firstborn, the infant received more attention from a caring mother, while later offspring were given less attention. Thus the firstborn would naturally feel more stress when less attention was paid to it, such as when parents leave or faced with a stranger. Its siblings, used to receiving less attention would show less stress.
Another explanation could simply be due to the age of the mother in question—younger mothers tend to be healthier, and giving birth to more alert offspring. As a general rule, first time mothers tend to be younger than experienced ones....