Sibling Rivalry: Are First-Born Children Smarter?

First-born children hold a mental edge over their younger siblings because they receive more mind-stimulating attention from their parents at a young age, finds a new study.

Assessing about 5,000 children who were born in the U.S. in the millennial generation, researchers from the University of Edinburgh in Scotland found that differences in IQ scores revealed themselves early on.

“As early as age 1, 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,” report the study authors in the Journal of Human Resources.

However, there doesn’t appear to be an innate intelligence difference, say researchers. Instead, the changing behavior of parents appears to have the greatest impact on their children — from first born and down the line.

“Our results suggest that broad shifts in parental behavior are a plausible explanation for observed birth order differences in education and labor market outcomes,” said Dr. Ana Nuevo-Chiquero, lead author of the study and professor with the School of Economics.

Related: Parents’ Obesity May Cause Severe Developmental Delays in Children

Researchers found that parents of multiple children routinely alter their treatment of non-first born children. For example, younger siblings were less likely to engage in mentally stimulating activities with their parents or alone, such as reading, doing arts and crafts, and playing musical instruments, according to the study.

Also, mothers may become less likely to remain as vigilant about their health when younger siblings come along. “Mothers take more risks during pregnancy and are less likely to breastfeed and to provide cognitive stimulation for latter-born children,” notes the study.

Comparing the New Results to Past Studies

The researchers tapped into a deep vein of data from the National Longitudinal Survey of Youth 1979 (NLSY79), which began tracking population health patterns among 14- to 22-year-olds in 1979. The current study looks at the health and developmental patterns of the children of NLSY79 parents, for which researchers have access to reporting data from 1986 to 2014.

The researchers also applied an assessment tool — the Home Observation Measurement of the Environment — to track patterns in the day-to-day lives of parents and children. The assessment tool analyzed behaviors among parents, such as smoking and drinking, as well as how much mental stimulation and emotional support children received.

Related: Complications During Pregnancy May Increase Autism Risk in Children

After close study, the researchers conclude that “variations in parental behavior can explain most of the differences in cognitive abilities before school entry.”

Other studies have tried to answer the question of birth order and later impact. For instance, a 2015 study found exceedingly small but steady associations between birth order and intelligence and personality.

Another large-scale study of nearly 380,000 U.S. high school students found that those born first have a one-point IQ advantage over their younger siblings — a finding that the study authors called “statistically significant but meaningless.”

The same study noted common personality traits among first-borns — they tend to be gregarious and conscientious and have less anxiety. But, again, the researchers cautioned that any differences in personality related to birth order are “infinitesimally small.”