According to several new studies, the first-born children get an IQ boost from having to teach their younger siblings and receiving undivided attention.
These differences in intelligence is thought to occur from nurture and upbringing, rather than biology.
A study by Leipzig University estimates a 1.5 drop in IQ points per sibling. However, in a two-child family, the eldest won’t necessarily dominate as they have a 60% chance of a higher IQ. This means the younger relatives still have a strong chance of overtaking them.
Alongside heightened intelligence, older children also display an increased perception of their brain power. The study’s data came from IQ and personality tests from three national studies, including one from the UK which involve more than 20,000 participants.
“While the firstborn gets full parental attention, at least for some months or years, late-borns will have to share from the beginning.”
Economists at the University of Edinburgh, Analysis Group and the University of Sydney examined survey data collected by the US Bureau of Labor Statistics and found that first-borns scored higher than siblings in IQ tests as early as age one.
The study, published in the Journal of Human Resources, observed nearly 5,000 children from pre-birth to age 14, with children assessed every two years.
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Researchers said the findings went some to way explaining the so-called birth order effect – which means children born earlier in a family have better wages and higher levels of education later in life.
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First-born children scored higher on tests including reading, matching letters, names, reading single words aloud and picture vocabulary tests.
Researchers also gathered information on environmental factors such as family background and economic conditions.
The study found parents changed their behaviour as they had more children, giving less mental stimulation and taking part in fewer activities like reading with the child, crafts and playing musical instruments. Mothers also took part in more risky behaviours such as smoking during pregnancy with subsequent children.