The Biological Roots of Intelligence
Shawna Williams

In 1987, political scientist James Flynn of the University of Otago in New Zealand found out something curious : intelligence had broadly increased in multiple human populations over time. The data was collected across 14 countries where many decades of average IQ scores of large fractions of the population were available. He found that all of them showed upward swings—some of them dramatic.
Children in Japan, for example, gained an average of 20 points between 1951 and 1975 on a test known as the Wechsler Intelligence Scale for Children. In France, the average 18-year-old man performed 25 points better on a reasoning test in 1974 than did his 1949 counterpart.1 Flynn initially suspected the trend reflected faulty tests. Yet in the ensuing years, more data and analyses supported the idea that human intelligence was increasing over time.
Proposed explanations for the phenomenon, now known as the Flynn effect, include increasing education, better nutrition, greater use of technology, and reduced lead exposure, to name but four. Beginning with people born in the 1970s, the trend has reversed in some Western European countries, deepening the mystery of what’s behind the generational fluctuations. But no consensus has emerged on the underlying cause of these trends.

What is intelligence?
A fundamental challenge in understanding the Flynn effect is defining intelligence. At the dawn of the 20th century, English psychologist Charles Spearman first observed that people’s average performance on a variety of seemingly unrelated mental tasks—judging whether one weight is heavier than another, for example, or pushing a button quickly after a light comes on—predicts our average performance on a completely different set of tasks. Spearman proposed that a single measure of general intelligence, called g, was responsible for that commonality.

What causes variations in g?
Scientists have proposed several biological mechanisms for variations among individuals’ g levels, such as brain size and density, and overall connectivity within the brain cortex. But the precise physiological origin of g still unknown. The study also becomes controversial because of eugenics, which attempts to label genetic groups as "superior" or "inferior" based on unethical practises.
Early studies of people with brain injuries indicated that the frontal lobes of the brain were vital to problem solving. In the late 1980s, Richard Haier of the University of California, Irvine, and colleagues imaged the brains of people as they solved abstract reasoning puzzles. As their brains were working, specific areas in the frontal, parietal, and occipital lobes of the brain, as well as communication between them, showed increased activity. The frontal lobes are associated with planning and attention; the parietal lobes interpret sensory information; and the occipital lobe processes visual information—all abilities useful in puzzle solving.
But more brain activity doesn’t mean better ability to understand and acquire knowledge: these are called cognitive prowess.  The people with the highest test scores actually showed the lowest brain activity, suggesting that it wasn’t how hard your brain was working that made you smart, but how efficiently your brain was working!

The parieto-frontal integration theory
In 2007, based on this and other neuroimaging studies, Haier and the University of New Mexico’s Rex Jung proposed the parieto-frontal integration theory. This states that the brain areas identified in Haier’s and others’ studies are central to intelligence.
But patterns of activation vary, even between people of similar intelligence, when performing the same mental tasks.  This can mean that there are different pathways that the brain can use to reach the same end point.

Models of intelligence
However, tools to study the brain are still limited. In addition, is intelligence only tied to the anatomical features of the brain?  It’s become clear over the past 10 to 15 years that this view is too simplistic.
Researchers have begun to propose alternative properties of the brain that might determine intelligence. Miller, for example, has been tracking the behavior of brain waves, which arise when multiple neurons fire in synchrony or together, for clues about IQ.
In one recent study, he and colleagues hooked up EEG electrodes to the heads of monkeys that had been taught to release a bar if they saw the same sequence of objects they’d seen a moment before. The task relied on working memory, that is, the ability to access and store bits of relevant information, and it caused bursts of different kinds of waves, some at high frequency and others at lower frequency. More importantly, when the bursts weren’t synchronized at the usual points during the task, the animals made errors. So not only are the frontal and parietal parts of the brain important, but the connections between them also determine your cognitive skills.

Network Neuroscience Theory
The overall pattern of brain communications is another candidate to explain intelligence. Earlier this year, Aron Barbey, a psychology researcher at the University of Illinois at Urbana-Champaign, proposed this idea, which he calls the network neuroscience theory.

Putting the g in genes
Is intelligence genetic? Can it be inherited from parents? It seems unlikely, since we know of lots of scientists whose brothers and sisters did not become famous! (Of course there are some such cases as well).  Some estimates state that about 25 percent of individual variation in intelligence will turn out to be genetic. While the correlation between education and intelligence is imperfect, “intelligence and school achievement are highly correlated, and genetically very highly correlated,” says von Stumm.
Despite the hints uncovered about how intelligence comes about, Santarnecchi finds himself frustrated that research has not yielded more-concrete answers about what he considers one of neuroscience’s central problems. To address that shortcoming, he’s now spearheading a consortium of cognitive neuroscientists, engineers, evolutionary biologists, and researchers from other disciplines to discuss approaches for getting at the biological basis of intelligence.
As researchers struggle with the phenomenon of intelligence, a philosophical question arises: Is our species smart enough to understand the basis of our own intelligence? While those in the field generally agree that science has a long way to go to make sense of how we think, most express cautious optimism that the coming decades will yield major insights.
Adapted from the article by Shawna Williams in The Scientist: https://www.the-scientist.com