Study finds firstborns are smarter - and there's a logical reason why

Oldest siblings are the smartest - science says so.
Oldest siblings are the smartest - science says so. Photo: Getty Images

A new study has demonstrated what us first-borns have always suspected: eldest kids are the smartest. And, as it turns out, it's likely due to the different parenting we received during those all-important baby years.

The research, published in The Journal of Human Resources, highlighted that from as early as 12 months old, firstborn children score higher on IQ tests than their younger siblings, when tested at the same age. According to the researchers, the finding may help to explain the documented birth order effect, where older children earn higher wages and obtain more education than their younger brothers and sisters.

As part of the study, a team from the University of Edinburgh, Analysis Group and the University of Sydney explored data from the U.S. Children of the National Longitudinal Survey of Youth, which was collected by the US Bureau of Labor Statistics. 5,000 children were observed from before their birth up until the age of 14 years, and tested every two years.

Information about participants' family backgrounds were also collected, including socio-economic details, whether or not mothers smoked or consumed alcohol during pregnancy, as well as levels of emotional support and mental stimulation provided to kids after they were born.

Results highlighted that while all children received the same levels of emotional support after birth, firstborns elicited more support from mum and dad with tasks that developed their thinking skills - ultimately giving them a cognitive edge. With subsequent children, parents also engaged in fewer activities such as reading, craft and playing musical instruments - part of what researchers termed a "broad shift in parental behavior" with latter-born kids.

Researchers also found differences in maternal behaviour for latter-born babies. "Mothers take more risks during pregnancy and are less likely to breastfeed ..." they note. More specifically, mums are less likely to cut down on their alcohol intake and tend to seek prenatal care at a later stage. 

Elaborating on their findings, co-author Jee-Yeon K. Lehmann told TODAY: "First-time parents tend to want to do everything right and generally have a greater awareness of their interactions with and investments in the firstborn."

As more children are added to their brood, however, Lehmann explained that mums and dads tend to relax, "what they might deem as non-essential needs for their kids."

For parents there's a clear message: those early investments in your kids matter.

"All those learning activities that you did with your first child as excited, nervous and over-zealous parents actually seem to have some positive, long-lasting impact on their development," Lehmann said.

Last year, a study using the same data set, found that what researchers termed "parental investment" (how often families eat meals together, how often parents display affection and how many books each child has) fell by around 3 per cent with the birth of each subsequent child.

"We find that families face a substantial quantity-quality trade-off: increases in family size decrease parental investment, decrease childhood cognitive abilities, and increase behavioral problems," the authors wrote of their findings.