It's also the same rationale behind the rude "construction worker" catcall. Although you can tell quite a bit about a person by their gait and attire, the false

It's also the same rationale behind the rude "construction worker" catcall. Although you can tell quite a bit about a person by their gait and attire, the false positive rate is high, so it's doubtful a man will learn enough just by watching to make it worth his while to approach a stranger in the street for a sexual encounter. But whistling and making lewd remarks instantly winnows out any "difficult" (which is to say assertive, intelligent or self-respecting) women, leaving the most willing and easily pliable as the ones most likely to respond positively.