First-born children are smarter than their younger siblings, according to a new study.
Economists at the University of Edinburgh concluded eldest siblings have a higher IQ test score than their brothers and sisters as early as age one.
Researchers suggested the findings could be explained by first-borns receiving more mental stimulation from their parents during their early years.
The study, published in the Journal of Human Resources, could also help settle the so-called birth order effect when older siblings in a family are said to enjoy better wages and more education in later life.
Almost 5,000 children were observed from pre-birth to 14 years old on their family background and economic conditions with assessments every two years on reading and picture skills as part of the research.
Data was examined from the US Children of the National Longitudinal Survey of Youth collected by the US Bureau of Labour Statistics.
The study was conducted in partnership with Analysis Group and the University of Sydney.
Dr Ana Nuevo-Chiquero, of the University of Edinburgh's School of Economics, said: "Our results suggest that broad shifts in parental behaviour are a plausible explanation for the observed birth order differences in education and labour market outcomes."
