Recent studies have revealed that there may be perceptible cues to ovulation in humans. This study aims at extending these findings by using female faces that were shape transformed towards a late follicular (fertile) and a luteal (non-fertile) prototype. Fertile prototypes were created by averaging 25 photographs taken of females during ovulation (as determined by ovulation tests); non-fertile prototypes were created by averaging 25 photographs of the same women during the luteal phase. Twenty different (new) female faces were then shape transformed towards the luteal prototype and towards the follicular prototype in 50% and 100% steps. The two 50% transforms and the two 100% transforms were paired, resulting in stimulus pairs of two different difficulties. Thirty-six male participants were asked to choose the more attractive (Task 1), the more caring (Task 2), and the more flirtatious face (Task 3). In a final task the participants were asked to choose the woman with which the participant would have better chances to get a date (Task 4). For all tasks we found a significant preference for the follicular face. In trials with a 100% transformation towards the shape of the prototype, the preference for the follicular stimulus was significantly stronger than in trials with a 50% transformation. We conclude that subtle shape differences in faces are sufficient to trigger men's preference for a woman in her fertile cycle phase.
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