Abstract
The effects of birth order on various dimensions of human personality and behaviour have been extensively studied. These effects might, however, be modulated by siblings' gender and their age-gap.
It was previously suggested that the greatest birth-order-related difference occurs among offspring with closer birth intervals and siblings with age-gap greater than 5 years are expected to affect each other rather negligibly. In the present research we focused on the effect of siblings' gender and their age-gap on educational achievements.
We collected data from biology students of Charles University in Prague (2011-2015) using a questionnaire containing family and birth order related questions. To avoid the confounding effect of family size, we selected only two-child families (N=781) and computed the proportions of firstborns while considering their gender and age-gap.
On average, we found a significantly higher proportion of firstborns irrespective of their sex and sex of their siblings. More importantly, the proportion of firstborns was highest among those whose younger siblings were approximately 1 to 3 years apart.
As the age-gap increases, the proportion of firstborns decreases in both sexes. However, the decrease is further modulated by sibling's gender and appears to be substantial in men as compared to women.
Our results highlight the importance of controlling for gender and age-gap among siblings and indicate that the frequently used 5-years age-gap might be misleading. We further suggest to employ the above reported approach for testing other psychological characteristics where birth order effect was previously reported.