Evidence for such negative pressure is inconclusive [1]. Such an effect may be more a result of socioeconomic status, not intelligence. As for a counter example of positive pressure on intelligence, I would expect the trait of humor [2], an attractive trait that is a by product of intelligence, to increase the probability of fertility.

1. http://en.wikipedia.org/wiki/Fertility_and_intelligence 2. Kaufman, Scott Barry, et al. "The role of creativity and humor in human mate selection." Mating intelligence: Sex, relationships, and the mind’s reproductive system (2008): 227-262.