According to the Trivers-Willard Hypothesis, individuals possessing a trait which improves the reproductive success of males more than females will be more likely to give birth to male offspring than to female offspring.  I came across a study that claims to support the hypothesis where the trait in question is promiscuity.

Our analyses of two large nationally representative samples, from the General Social Survey in 1994 and the National Longitudinal Study of Adolescent Health, confirm this prediction.  Controlling for a large number of social demographic factors that might be expected independently to influence offspring sex ratios, unrestricted sociosexual orientation significantly increases the odds that the first child is a boy.  One standard deviation increase in the unrestrictedness of sociosexual orientation increases the odds of having a son by 12-19%.

That seems like a large effect, if true.  Chullo chuck:  Barking Up The Wrong Tree.