Mate poaching...

Pretty interesting:

Noting that single women often complain that “all the good men are taken,” the psychologists (Melissa Burkley and Jessica Parker of Oklahoma State University) wondered if “this perception is really based on the fact that taken men are perceived as good.”

[...E]ach of the experimental subjects was told that he or she had been matched by a computer with a like-minded partner, and each was shown a photo of an attractive person of the opposite sex. (All the women saw the same photo, as did all the men.) Half of the subjects were told that their match was already romantically involved with someone else, while the other half were told that their match was unattached. Then the subjects were all asked how interested they were in their match.

To the men in the experiment, and to the women who were already in relationships, it didn’t make a significant difference whether their match was single or attached. But single women showed a distinct preference for mate poaching. When the man was described as unattached, 59 percent of the single women were interested in pursuing him. When that same man was described as being in a committed relationship, 90 percent were interested.

According to a recent poll, most women who engage in mate poaching do not think the attached status of the target played a role in their poaching decision, but our study shows this belief to be false. Single women in this study were significantly more interested in the target when he was attached. This may be because an attached man has demonstrated his ability to commit and in some ways his qualities have already been ‘‘pre-screened” by another woman.

There was also this study:

Are all the taken men good? An indirect examination of mate-choice copying in humans - Kevin W. Eva, Timothy J. Wood - 2006

Across many species, females are typically the more choosy of the 2 sexes because female-specific investment (e.g., gestation and lactation) constrains the number of offspring a female can produce, whereas the main constraint in this regard for males is simply access to females.2 As a result, poor mate choices harm a female's reproductive value to a greater extent than they do a male's. Evolutionary theory predicts that such pressures will lead females to rely on cues from their environment that can aid them in determining the reproductive value of a potential mate. One cue shown to be used by female Japanese quail and numerous other species is the choice behaviour of other females.3 If a female has deemed a male worthy of mating, that provides information to other females about the value of that male. Whether or not human females are sensitive to such social information (mate-choice copying) is, therefore, an interesting empirical question. [...]

Females were simply asked to rate the attractiveness of males who were presented in head-and-shoulder photographs. Each male was presented to half of the participants as “married” and to half as “single.” Analysis of covariance revealed that males were rated as more attractive when labelled “married,” thereby suggesting that human females are indeed sensitive to information provided by the choices of other females, despite the minimalist nature of the intervention used in this experiment.