Identical twins who were raised apart may have IQ differences similar to those of total strangers, according to new research. The findings suggest that variations in IQ may be less about genetics and more about schooling.
The heartbreaking separation of twin siblings is a rare occurrence, and only nine large group studies have been published to date.
In the past, researchers have concluded that identical twins raised apart have many matching traits, including similar IQs, suggesting that IQ (a sign of intelligence) is largely determined by nature, not nurture.
Related: Identical Twins Aren't 100% Genetically Identical After All, Study Finds
Not so fast, argue cognitive neuroscientist Jared Horvath and developmental researcher Katie Fabricant. These two have crunched the numbers again, and this time, they've included a key overlooked factor: schooling.
When the researchers divided 87 twin-pairs into groups based on similar and dissimilar schooling backgrounds, they found IQ differences across the spectrum.
The gaps in IQ scores grew in tandem with educational differences, the authors say, "enough to transcend specific teachers or peer groups."
Twins that were raised apart and who went to significantly different schools showed IQ patterns more similar to strangers (a roughly 15-point difference).
There were only 10 twin-pairs in the study with school experiences that met suitable criteria, making for a small sample size that places limits on the study's conclusions.
The IQ test was developed in 1905 to determine which kids needed extra help in school.
Over the decades, studies have suggested that IQ scores remain consistent over a person's life, a fact that has been used to argue that some forms of intelligence are baked in. In other words, they are more due to nature than nurture.
Across the generations, however, IQ scores seem to be climbing.
Growing evidence now suggests that improved education is causing a consistent increase in IQ across various nations and cultures.
The extent to which schooling can impact IQ is challenging, if not impossible, to study in isolation.
Identical twins are often used to weigh in on nature versus nurture debates, but these siblings are often raised together, and even if they are raised in different households, they sometimes attend the same schools.
"There is a lot more work to be done in order to map the entirety of environmental influences on cognitive tests," admit Horvath and Fabricant.
"Hopefully, this paper can serve as a brick in the larger wall of understanding being built."
The study was published in Acta Psychologica.