Abstract
For many mammals, siblings are an important part of the developmental environment, probably contributing to the formation of individual differences in personality. Specifically, in humans sibling effects are thought to be associated with birth order. However, the findings have been controversial. This might be due in part to the use of self-report methods, which depend on subjective perceptions. Thus, it is necessary to also use physiological measures not under voluntary control. We tested the behavioral and physiological responses of 80 young adults—40 men and 40 women, 20 firstborn and 20 lastborn of each sex—to 3 commonly used psychological stressors during a 13-min experimental session. Participants’ behavioral responses were recorded on video, and their physiological reactions were registered via facial infrared thermography. As predicted by birth order theory, firstborns showed significantly greater signs of stress than lastborns, with lower nose temperatures on all tests and for both sexes. Behavior did not differ between the 2 groups. Thus, contradictory findings among previous studies of the association of birth order with personality differences between siblings might be partly due to the low resolution of behavioral evaluation in contrast to the registration of autonomic nervous system activity as, for example, measured by noninvasive, easily implemented facial infrared thermography.
