First-born children get more attention from their parents which leads to better results in life, a study has suggested.
From as early one year old, researchers found that the first child scored higher in IQ tests.
Economists at the University of Edinburgh, Analysis Group and the University of Sydney believe this is a result of greater mental stimulation in their early years.
Although all children received the same levels of emotional support, first-born children received more support with tasks that developed thinking skills, the study over 14 years showed.
Researchers believe the findings might explain the so-called birth-order effect, where children born earlier in a family enjoy better wages and more education in later life.
Nearly 5,000 children were observed from pre-birth to age 14. Each child was assessed every two years.
The tests included reading recognition, such as matching letters, naming names and reading single words aloud and picture vocabulary assessments. Information was also collected on environmental factors such as family background and economic conditions.
The findings showed that advantages enjoyed by first born siblings start very early in life – from just after birth to three years of age.
The differences increased slightly with age, and showed up in test scores that measured verbal, reading, math and comprehension abilities.
Researchers found that parents changed their behaviour as subsequent children were born. They offered less mental stimulation to younger siblings also took part in fewer activities, such as such as reading with the child, crafts and playing musical instruments.
Mothers also took higher risks during the pregnancy of latter-born children, such as increased smoking.
Dr Ana Nuevo-Chiquero, of the University of Edinburgh’s School Economics, said: “Our results suggests that broad shifts in parental behaviour are a plausible explanation for the observed birth order differences in education and labour market outcomes.”