Birth Order and Intelligence in the Children of Families with Intellectually Disabled Child

Research indicate the relationship between birth order and intelligence quotient (IQ), generally suggesting that the first-borns have a higher IQ. This study aims to investigate the relationship between birth order and IQ in the children of families with intellectually disabled children.

This research was a fundamental study in terms of its goal and a descriptive and casual study in terms of gathering data. The study population included all the families with intellectually disabled children in Tabriz in 2016. The sample included 194 families selected by cluster sampling from this population. Data were collected by researcher-made questionnaire and Ravenchr('39')s intelligence test. Correlation and One-way ANOVA were used to analyze the data.

Data analysis showed that there was no significant difference in the birth order and IQ in families with three and four children, whereas the difference was significant in families with five children (P<0.04). Also, results showed that there was a significant relationship between the birth order and IQ of the children, in such a way that the first-borns have the higher mean IQ, while second-born and third-born children have the lowest IQ.

In short, contrary to research literature and birth order theories that claim superiority of intelligence in first-born children, this research did not establish such a relationship. Results showed that birth order effects are a function of family size and birth order has no pure effect on the intelligence of children.