Economists at the university say the findings could help explain the so-called birth order effect when children born earlier in a family earn higher wages and do better in education in later life.
Children born first in the family scored higher in IQ tests as early as age one.
Although all children received the same emotional support, first-borns received more support with tasks that developed thinking skills.
Nearly 5,000 children were observed from pre-birth to age 14, and assessed every two years.
Dr Ana Nuevo-Chiquero, of the university’s school of economics, said later-born children needed extra attention.
“Parents say ‘I treat all my kids equally but first-borns are the only child in a family for a while and get a lot of attention with parents showing extra willingness in reading to them and so other activities which tend to make their child smarter. Parents need to be aware of this and pay extra attention to the younger ones, doing more activities with them.”
Researchers observed parental behaviour, pre and post birth, including smoking and drinking during pregnancy.
Researchers found parents changed their behaviour with subsequent children, offering less mental stimulation. Younger siblings also took part in fewer activities such as crafts and playing musical instruments.
The study, published in the Journal of Human Resources also involved Analysis Group and the University of Sydney, examining data from the US Children of the National Longitudinal Survey of Youth.