The authors performed a meta-analysis of the literature, focusing only on studies that measured cognitive abilities in people who self-identified as chess players or not. This design did not allow the researchers to determine causation (e.g., playing chess makes a person smarter, or being smart causes a person to be good at chess).
However, it did allow the researchers to ask, “Are people who play chess by their own free will more intelligent than people who do not?” If the answer to that question is yes, then that provides evidence that the “access to training” hypothesis for expertise is wrong. Furthermore, it provides indirect evidence that intelligence drives people toward certain fields and plays a role in how successful they are in those fields*.
After screening 2,287 papers, seven met the authors’ criteria for inclusion. Combined, the studies involved 485 people. The results of the meta-analysis are shown below.
The average effect size (indicated by the diamond on the lower right) was d = 0.49, which means that, for whatever cognitive trait the study was analyzing (problem-solving, verbal ability, memory, creativity, etc.), chess players scored on average half of a standard deviation higher than non-chess players.