First-born children do better at school because parents pay them more attention, according to new research.
The extra focus gives them an “edge” over younger brothers and sisters and higher IQs - as early as the age of one, suggests the study.
Researchers found the eldest child outperformed siblings in thinking skills after receiving more “mental stimulation”.
Advantages started from just after birth to three years old. The differences were highlighted in language, reading, maths and comprehension abilities.
As subsequent children were born mums and dads changed their behaviour - taking part in fewer activities such as such as reading, crafts and playing musical instruments.
Mothers also took higher risks - they were more likely to smoke during pregnancy once they had already had a child and were also less likely to breastfeed after birth, for instance.
Dr Ana Nuevo-Chiquero, of Edinburgh University, said: “Our results suggest broad shifts in parental behaviour are a plausible explanation for the observed birth order differences in education and labour market outcomes.”
They add to the long-running debate about why first-born children are typically more successful.
Previous research has revealed younger children do less well in terms of overall educational attainment than their older brothers and sisters.
A first child is typically at least a year ahead of a third-born brother or sister at the equivalent stage at school.
The latest study, published in the Journal of Human Resources, showed all children received the same levels of emotional support.
But first-borns got more help with tasks that developed thinking skills.
Dr Nuevo-Chiquero said the findings could help to explain the ‘birth order effect’ phenomenon when children born earlier in a family enjoy better wages and more education in later life.
She said: “It doesn’t mean first-borns get more love - that stays the same. But they get more attention - especially in those important formative years.
“As the household gets bigger time has to be split with younger children so they miss out on the advantage of being an ‘only child’ for a time.”
Mothers could become complacent after giving birth to their first baby and change their health behaviours.
She said: “As early as age one, latter-born children score lower on cognitive assessments than their siblings, and the birth order gap in cognitive assessment increases until the time of school entry and remains statistically significant thereafter.
“Mothers take more risks during pregnancy and are less likely to breastfeed and to provide cognitive stimulation for latter-born children.
“Variations in parental behaviour can explain most of the differences in cognitive abilities before school entry.”
Dr Nuevo-Chiquero said numerous studies have found large differences in the education and labour market outcomes of adults by their birth order.
But this was the first to compare first-borns with younger siblings from the womb through childhood - and it could provide important lessons for parents.
She said: “For most, it is probably not difficult to understand how and why one’s parenting focus and abilities may change with his/her latter children.
“These broad shifts in parental behaviour appear to set their latter-born children on a lower path for cognitive development and academic achievement with lasting impact on adult outcomes.”
Her team followed American youngsters from pre-birth to the age of 14 using data from the US Children of the National Longitudinal Survey of Youth.
Every child was assessed every two years with tests including 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.
Researchers applied statistical methods to economic data to analyse how the parental behaviour of the child was related to their test scores.
The researchers then used an assessment tool called the Home Observation Measurement of the Environment to look at parental behaviour.
This included smoking and drinking while babies were in the womb and mental stimulation and emotional support after they were born.
In 2013 a survey found 33.8 per cent of mothers claim their first-born is ‘one of the best students in the class’ - and only 1.8 per cent put their children at the bottom.
With each successive sibling, the former fell and the latter figure rose: essentially, more mothers consider their second or third born children less intelligent.
Of the women surveyed, 31.8 per cent said their second born was one of the best class.
Less than a third, 29 per cent, said the same of their third, and 27.2 per cent rated their fourth in the top.
Conversely, the ‘near the bottom of the class’ numbers rose: two per cent rated their second the worst, 2.1 the third, and 3.6 the fourth.
It was suggested parents are more likely to bring up their first children strictly, while younger children are less likely to be punished if they get bad marks.
Another study estimated in a two-child family the eldest will have the higher IQ six times in 10 - a statistic that still gives younger children opportunity to shine.