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The sex difference in marathon performance increases with age and place of the finisher, even at the elite level. Sociological factors may explain the increased sex gap, but there is limited empirical evidence for specific factors.The purposes of this study were to determine the sex difference in velocity for the marathon across the place of finisher (1st–10th place) with advanced age and (2) to determine the association between the sex difference in participation (ratio of men-to-women finishers) and the sex difference in running velocity.Running times of the first 10 placed men and women in the 5-yr age brackets between 20 and 79 yr and the number of men and women who finished the New York City marathon were analyzed for a 31-yr period (1980–2010).The sex difference in running velocity increased between the 1st and the 10th place because of a greater relative drop in velocity of women than men (P < 0.001). The sex difference increased with advanced age and decreased across the 31 yr, but more for the older age groups (P < 0.001). The number of women finishers also increased relative to men for the 31 yr, but more in the older age groups (P < 0.001). Importantly, approximately 34% of the sex difference in velocity among the first-place finishers was associated with the ratio of men-to-women finishers (r = 0.58, r2 = 0.34, P < 0.001).The greater sex difference in velocity that occurs with age and with increased place was primarily explained by the lower number of women finishers than men. These data provide evidence that lower participation rates and less depth among women competitors can amplify the sex difference in running velocity above that due to physiological sex differences alone.