We examined the effect of nectar robbery on phase duration of a natural population of Impatiens capensis Meerb. (Balsaminaceae), a species with protandrous flowers. We hypothesized that once robbed, a male‐phase flower should switch its sex from male to female because the probability that the flower would receive enough visits to disperse all its pollen is low compared with the probability that the flower would receive enough visits to fertilize its three to five ovules. We assumed that robbery results in a greater reduction in the attractiveness of male‐phase flowers relative to female‐phase flowers. Field observations supported our hypothesis and its assumptions by demonstrating that natural nectar robbery significantly shortened the duration of male phase. A field experiment in which flowers were experimentally pierced with needles (=flower piercing) showed that the physical damage to the flower caused by piercing alone shortened the duration of male phase. In both the observational and experimental st...