Why are men sometimes better than women at chess?

Chess tournaments are generally open to all participants, regardless of gender. There are a few international chess tournaments for men only, and a few long-term tournaments for women only, notable among them being the Women's World Chess Championship and the Women's Chess Olympiad. The World Junior Chess Championship and the World Youth Chess Championship include simultaneous tournaments for girls in various age groups. Some countries hold a separate national women's championship in addition to the national chess championship, which is open to both genders. 
A statistical analysis of German player ratings in a 2009 article by Miriam Bilalic, Kieran Smallbone, Peter McLeod, and Fernand Gobet indicated that although the top-ranked men in chess were stronger than the top-ranked women (usually by more than 200 points), this difference was largely due to the fact that the numbers of women players included in the statistics were relatively lower than those of male players (only one in sixteen of the rated German players were women).
In 2020, psychologist and neuroscientist Wei-Jie Ma summarized the lack of female chess players by stating that there is currently no scientific evidence of biological differences between the sexes in chess-playing ability, but this does not mean that such differences do not necessarily exist.
Reasons for gender differences in chess rankings
Some male and female chess players have speculated about the reasons for the gender gap in chess statistics. Some female players believe the main reason is cultural expectations and bias. Jennifer Shehad, the director of the women's player program at the US Chess Federation, stated that a large number of 12- and 13-year-old girls in the US Chess Federation do not receive enough support to continue advancing in the game due to the lack of a supportive social network for girls of that age to play chess. Polgar stated that society and some parents may discourage young female chess players from improving, and that low ambition often hinders women, leading them to choose to compete in women's tournaments over open tournaments for both sexes. Jovanka Houska, an international teacher and grandmaster, suggested that boys' overconfidence gives them an advantage over girls. A 2007 study conducted at the University of Padova compared male and female players of similar ability in online chess games. They found that when players were unaware of their opponent's gender, female players won fewer than half of their matches. When female players were told their opponent was male, they played less aggressively, winning about one in four matches. However, when female players were told their opponents were female, despite them actually being male, they played just as aggressively as male players, winning about one in two matches. The researchers suggested that gender stereotypes prevalent in society may have led female players to lower their self-esteem and confidence when they knew they were playing against male players, causing them to play defensively, leading to a decline in their performance.
Some male players have commented on the performance of female chess players. In a 1963 interview, Bobby Fischer rejected the idea of gender equality in chess, describing female players as "horrible," claiming they were not intelligent. Grandmaster Nigel Short also suggested in 2015 that male players performed better because men and women had different skill sets, a view that was met with considerable controversy in chess circles.