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.

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Supposedly a man's brain is slightly different than a woman's brain. Men are supposedly better in spatial reasoning and women are better with verbal skills. However, the brain is extremely trainable. With practice any skill can be developed. I think women suffered from more social biased which makes it more challenging to learn the proper skills for chess. In the end, if all factors were the same, I think both men and women would play the same.

I think gender makes no difference in chess skill

This is true. Men tend to orient themselves in terms of direction, whereas women are more in terms of landmarks.

Well men are not smarter than women, and i have been beaten by young ladies in chess many times.

However women are lots prettier than men for sure.

Men are not. It's just that men have had more opportunity to succeed in history, while women were seen as merely housewives.

..., I prefer to say is biological but, men and women have the same opportunity to play and learn chess in any sense, is just that men had more presence on Chess history. On these days the Titles of masters between men and women are kinda different but isn't because their capabilities, is for a different title level on their own. For example: a FM isn't the same as a WFM.

Yes they are but it’s because there are so many more men who play chess than women.

I have read statistics showing that the average rating of males and females is about the same. This would suggest that men are not better at chess than women.

It used to be that women were only about 5 percent of the tournament pool. (I don’t know what the current stat is.). If this is so, then there should be 19 male masters for every female master. Isn’t this about the ratio of male to female GMs?

Women are better at chess. The Queen's Gambit proved that.

The Queen’s Gambit isn’t based on a true story or a documentary. 🙂

In 1990 the ratio, by gender, of actively competing chess GMs was about 215:1 male to female. By 2000 that had dropped to about 78:1, by 2010 it was 52:1, today (2025) its about 43:1. World-wide, in 2025, membership in national chess organizations still strongly skews male ,approximately 90% of competitive chess players are male,however,60 years ago it was a statistical zero. As women have been given the opportunity and social encouragement to engage in chess, the number of elite female players has grown to reflect the changing demographics of chess. The pool of elite women players will at some point, in western, liberal nations at least, rounding error aside, proportionally match the gender distribution in chess, and the gender-gap for GM's will disappear.

If I recall (my memory might be off a bit), didn't Fischer change his mind after meeting and spending time with the Polgars?

Nigel Short also believed that some random engine user who clobbered him online was secretly Bobby Fischer - which suggests that Short's critical thinking skills weren't exactly up to par ...

I believe a better argument came from an exchange between Hou Yifan and Kramnik. Kramnik suggested that Yifan could've been even stronger (a super grandmaster) if she devoted more of her life to chess, and stopped pursuing other things. Yifan countered that there are more important things to life than just chess.

Personally, I agree with Yifan. But, we all have our own perspectives.