In one experiment, participants were asked to imagine that they had an attractive coworker who they tried to talk to every morning. The attractive coworker either displayed a low level of reciprocity (only engaging in conversation once a week), a moderate level (three days a week) or a high level (every day, and they wait outside work, join other conversations and phone at night).
The ratings of attractiveness for the coworker, on a scale of 1 to 7, were 3.7, 4.6 and 3.3 for the low, moderate and high levels respectively. So play it cool, but not too cool.
Similarly, a separate paper found that mates displaying moderate levels of availability were the most desirable. Another study found that women rated men as more attractive when they didn’t know how attractive the men found them.