A lot of what marie posted about is true with my bird, Scrubby.
My partner and I got her from a female breeder, so when she first came home, we would need to speak to her in high-pitched tones to encourage her. Otherwise, she would be shy. Since she came to live with us though (and we were able to use our normal speaking voices), she only ever interacts with men.
When women come over, they either are too afraid of Scrubby to touch her or they pull their finger away at the wrong moment, and Scrubby steps up into air. So, after being "faked out" too many times, in the past month we've seen her nip women on two occasions when she's never nipped before. Male guests are completely different though. She'll happily leave my shoulder to perch on someone she's never met as long as it's a guy.
We have a house guest right now who is female, and Scrubby nipped her even though she moved her finger in smoothly, confidently, and at a moderate speed. After observing Raquel (our friend) for a couple days, though, Scrubby became more comfortable and climbed up onto her shoulder.
Unfortunately, after being nipped, Raquel is suspicious of Scrubby planning an attack, so it'll be a while before they're friends