Spatial ability is a bit of a fuzzy concept. There are lots of different tests that can be used to assess it, like mentally rotating a 3D shape or reasoning out how a mechanical object would work. Are all of these tests really looking at the same thing, or are they probing unrelated capacities that we’re artificially combining under the heading of “spatial ability?"

A paper in PNAS this week suggests that different spatial tests are all basically testing the same underlying ability—and that this ability is only partly explained by general intelligence. This means that spatial ability is, to some extent, independent: you can have better (or worse) spatial ability than your general intelligence might suggest. The results also suggest that about a third of the differences in people's spatial scores can be explained by genetics.

Your genes don't have the last word

For any given trait that people have, both genes and environment will play a role. The tricky part is working out how much of a role each can claim. Take height, for instance: if everyone has enough food when they’re growing up, the role of environment will be limited, and so people’s heights will be determined mostly by their genetics—maybe as much as 80 percent. In a more unequal society, someone may have the genes to be very tall but not get enough food as a child, and this person will end up much shorter than they might have been. In this more unequal environment, genes can explain much less of the differences between people.

One of the most well-established methods for estimating the role of genes is studying twins. Pairs of twins are raised in very similar environments; there are obviously some differences, but they're smaller than they would be between unrelated people or siblings of different ages. Not only do they share environments, but identical twins share all of their genes (fraternal twins are like any other siblings, sharing roughly half of their genes).

If identical twins are more similar in some way compared to fraternal twins, we can probably pin this on their genes. In the case of height, a pair of identical twins (who will eat much the same food growing up) will probably end up very close in height; fraternal twins will have the same food but will probably be a bit different in height, because their genes aren’t identical.

The King's Challenge

A team of researchers led by Kaili Rimfeld of King’s College London studied more than 1,300 pairs of twins to figure out to what extent genes contribute to spatial ability. But first, they had to figure out which aspect of spacial reasoning to test them on. The researchers scoured the scientific literature to find all the different tests that had been used to assess spatial ability and ran pilot studies using them. They took out any tests that were too easy or difficult and any tests where the same people scored inconsistent results when taking the same test twice. The team also looked at the similarity of people’s scores in different tests—if the scores were very similar, they took out the redundant tests. By doing this, they boiled spatial ability down to 10 core tests.

The team used these 10 tests with the twin sample. People did get different scores for different tests, but the scores correlated with each other. Overall, 42 percent of the differences in the scores could be explained by a general spatial ability. The researchers tried other explanations for the data, looking at combinations of more than one kind of spatial ability, but these combinations didn’t explain the data as well as the one unifying factor. Obviously, there are still big differences in the scores that aren’t explained by this factor, but it’s a big chunk.

Untangling the causal spaghetti

All of the participants had been enrolled in the Twins Early Development Study since birth. Because of this long history, Rimfeld and her team also had data on the participants’ general intelligence scores from the ages of seven to 16. The results of the spatial tests overlapped somewhat with general intelligence—but not entirely. In this case, assuming two different abilities explained the data better than assuming one overall kind of intelligence.

Comparing the results of the identical and fraternal twin pairs found that 69 percent of the differences in spatial test results could be explained by genetic similarity. Of the remainder, the majority—23 percent—was explained by individual experience. That only leaves a small bit of ability to be explained by the environment that the twins shared. The researchers emphasize that these estimates are unique to this population: in a less equal environment than the UK, genes might explain less of the difference.

The researchers also compared the genetic overlap with general intelligence. They found that after controlling for general intelligence, 30 percent of the differences in spatial scores could be attributed to genetic differences.

Spatial ability is important, the authors write, because it’s one of the factors that contributes to achievement in STEM (science, technology, engineering and mathematics) fields. Understanding it better, and being able to assess it more easily, could make it easier to identify gifted children. Screening programs that identify talent early have been found to increase the number of minority, female, and underprivileged students in gifted programs.

Easier testing could also help students at the other end of the spectrum. “It would be a mistake to interpret weak shared environmental influence... as evidence that training spatial ability is not possible,” the authors write. Current research suggests that training can have a substantial effect on spatial ability, but the twins in this study probably weren’t exposed to this sort of training, so it wouldn't show up in their results. If teachers can assess children who are struggling, it could make it easier to give them support early on.

PNAS, 2016. DOI: 10.1073/pnas.1607883114 (About DOIs).