An interesting study from the journal Psychological Science presents data that suggests that married women are attracted to single men during the fertile period of their menstrual cycle and conversely attracted to partnered men when least fertile. Single women did not show this cycle of preference.
When coupled women were asked to think about having an affair, they chose partnered men over single men when they were in the fertile part of their menstrual cycle suggesting a subconscious mechanism at work.
Genetic makeup determines attractiveness
They suggest that married women during their fertile period are attracted to men with good genes but less emotional attachment and child-rearing skills. Men with better genetic makeup tend to be poorer partners and parents than men of lower genetic quality and so women may enter a steady relationship with the latter and have an affair with the former.
This theory argues that females view males with good genes as so desirable, that to pass this high genetic quality along to their kids, they are willing to give up help with the child-rearing and support. Males of lower genetic quality may make up for their poorer quality genes by being supportive and helping in the child rearing. Good genetic quality was perceived by women as men who had strong male characteristics and appearance.
Less fertile women look for investment by potential partners in child-rearing
The inherent drive to conceive a child during the fertile part of the menstrual cycle makes married men less attractive because they are less available for impromptu sex. When at the least fertile point, women tend to look at the level of investment in a long-term relationship by men. So already partnered men appear more attractive as they have already demonstrated their willingness to commit to a long-term relationship.
See Why married men are more attractive from more information on this
Surveying more than 200 women, half of whom were single, the other half in relationships, the women were asked to rate the attractiveness of men in photographs described as single, married, in love, or with a girlfriend. When the same man was labelled as single rather than being in any kind of attachment, fertile women in relationships scored single men 13% higher than attached men; infertile women scored them 8% lower.
Fertile women having affairs may instinctively be looking to get pregnant
The study raises the possibility that women in committed relationships who have an affair with a single man may be instinctively looking to get pregnant. Luckily our behavior is not completely ruled by instinct or our subconscious and our actions are the result of a much more complex interplay of influences.
Paul Nyhan's Working Dad: An Unauthorized Guide to Parenting blog also has an interesting commentary on this published study.