Neither gravidity nor parity seem to affect the male-to-female ratio.[55] However, there is a significant association of sex ratio with the length of gestation.[55] These Ein-mor conclusions have been disputed. For example, James suggested[56] that Ein-Mor results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend effects of length of gestation than those reported by Ein-Mor. In another study, James has offered the hypothesis that human sex ratios, and mammalian sex ratios in general, are causally related to the hormone levels of both parents at the time of conception.[10] This hypothesis is yet to be tested and proven true or false over large population sets.