One new study links IQ with the quality and quantity of one’s education, and another to a child’s home environment.

Is a person’s intelligence innate and unchanging, or something that can stagnate or grow depending upon the environment in which it is nurtured? Few scientific controversies have been fought more fiercely.

Twenty years ago, the best-selling book The Bell Curve argued that inherited ability was all-important, while critics responded by insisting on the importance of intellectual stimulation.

With its real-world applications for public policy, this dispute is anything but academic. Do inner-city kids score poorly on IQ tests because their learning capacities are limited (an argument many decry as racist)? Because the tests are biased? Or because their schools, and in many cases their home environments, are not conducive to the development of their innate abilities?

While conceding that IQ has a genetic component, two newly published studies provide evidence disproving the static-intelligence idea.

Intelligence is “formed through an interaction between genetic endowment and environment,” and that education “can be a fundamental factor in the environment’s influence.”

“Increasingly, it makes less sense to think of genes and environments as independent causes,” writes a research team led by Penn State sociologist David Baker. Its examination of likely reasons for the gradual rise in IQ scores over the 20th century suggest more challenging curriculums, to a significant degree, create smarter students.

“Despite being demonstrably related to genetic endowment, cognitive ability is environmentally malleable,” agrees a research team led by Kenneth Kendler of Virginia Commonwealth University. Its look at Swedish siblings finds higher IQs among those whose adoptive parents were better-educated.

Writing, appropriately enough, in the journal Intelligence, Baker and his team describe a three-pronged study that finds consistent evidence of the impact of education on intelligence. They begin by noting that “mean IQ test scores of cohorts of American adults increased by approximately 25 points over the last 90 years.”

This, they point out, coincides with a huge increase in the percentage of American youth who are enrolled in school, and the number of years they spend there. Just as importantly, perhaps, their careful examination of primary-level math instruction books found “the overall cognitive demands of textbook material increased significantly” over the decades.

“Starting in the mid-1960s, and increasing through the remainder of the century, young students were taught with material necessitating more effortful reasoning approaches to arithmetic,” the researchers report.

This is particularly interesting, they write, in the context of the brain scans that suggest “similar neurobiological mechanisms" underlie the solving of math problems and a range of "cognitive-executive functions” that provide mental resources for planning, organization, and "skills for goal-directed behavior."

These functions, they note, are widely considered “the foundation for domain-general intelligence.” It seems solving math problems doesn’t just make you good at math.

Another of their experiments, which featured residents of isolated agrarian communities in Peru, found that "variable exposure to schooling is associated with related variation in mental abilities.”

This was particularly telling, since all participants “reported starting and stopping their schooling because of the vicissitudes of subsistence-level farming, finances, access, or gender preferences,” the researchers report. “Pre-school cognitive endowments” were a non-factor, meaning the instruction raised intelligence across the board, regardless of innate abilities.

“The neurobiological plasticity underlying human cognitive development is robust enough to thrive despite many biological and environmental hazards,” the researchers conclude. “But at the same time, there is increasing evidence that such development is inherently tethered to one’s immediate environment, (which) shapes the brain in specific ways.”

Baker and his colleagues argue that these results challenge the basic premises of The Bell Curve. Their study suggests that intelligence is “formed through an interaction between genetic endowment and environment,” and that education “can be a fundamental factor in the environment’s influence.”

As can one’s home environment. That was confirmed by Kendler’s study, which was published in the Proceedings of the National Academy of Sciences. It took advantage of the fact that the IQ of all young Swedish men is measured at age 18, 19, or 20 as part of their mandatory examination for military service.

These researchers looked at 436 sets of siblings, all males, “in which at least one member was reared by one or more biological parents, and the other by adoptive parents.” They found the adopted siblings, on average, had an IQ 4.4 points higher than their brothers.

“Offspring placed in the best-educated homes had the highest scores,” they write, “whereas those placed in homes less educated than the family of origin actually performed worse than their non-adopted siblings.”

So, far from being set at birth, IQ appears to be influenced by both one’s home environment, and the quantity and quality of schooling one receives. To fully develop, budding geniuses need fertile soil.

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.