Lass-Hennemann et al. make the interesting observation that men, who generally prefer mates that resemble themselves, switch to preferring dissimilar mates after they are subjected to a very simple stress procedure (holding their right hand in ice water for three minutes). This conforms with research in animals showing that individuals under stress are willing to mate with a wider variety of partners. Attraction to the images shown was measured by an interesting technique, noting startle eyeblink responses to a burst of white noise. Those in a more upbeat frame of mind (i.e. feeling attraction) flinch less. Scratching around for a possible evolutionary rationale for this behavior, one suggestion is that when conditions seem favorable men have the option of picking physically similar mates, whom they subconsciously deem dependable partners who will nurture their offspring. But in stressful times, during which human survival is less guaranteed, men would be more willing to risk a physically dissimilar partner in order to father as many children as possible.
Here is a figure describing how the stimulus photographs were morphed:
Image editing procedure: the nude woman's detailed face (1) was morphed with the portrait picture of the participant (2). The morphing software produces two output images, a shape-only morph (3a) and a combined shape-colour morph (3b). In a second step, the shape-colour morph is used as a semi-transparent layer on top of the shape-only morph. All artefacts of the morphing procedure are eliminated. The resulting image (4) was photo-mounted on the woman's body in a last step (5). The resulting image was used as a stimulus (the image was not masked in the experiment).
I hypothesize that is it not a function of similarity, but familiarity. For most people the two are congruent, but controls should be done with people who were adopted or who have adopted siblings.
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