Finally, we addressed the participation-rate hypothesis. If in the general population the number of boys who play chess is substantially larger than the number of girls, the best ones ultimately becoming USCF members and playing competitively, then it follows statistically that the average boys' ratings will be higher than the average girls' ratings (among competitive players) even if the distribution of abilities in the general population is the same (Charness & Gerchak, 1996; Glickman & Chabris, 1996). In fact, far fewer girls than boys enter competitive chess, which suggests that the general population of chess-playing girls is much smaller than that of boys. External factors like the relative lack of female role models among the world's top players and the prospect of playing a game dominated by boys may be discouraging to girls (or their parents), either directly reducing their likelihood of learning how to play in the first place or indirectly reducing their initial performance in competitive play via test anxiety or stereotype threat (Steele, 1997). Thus, it is possible that, on average, girls have the chess-relevant cognitive abilities, but the larger number of boys playing chess leads to significantly higher male ratings in the USCF population.
Jake Young of the Pure Pedantry blog has a nice and succinct summary of the report.