You're six months into your new relationship. One of your friends told you about some research where they're looking for couples. Now you're here, in a university in Poland, having your photo taken. Yesterday, your boyfriend was here having his photo taken. And, in a few days' time, his mother is going to come to the university to have her photo taken. Yes, you should already be worried.
At the same time as this is going on, the researchers are busy recruiting 'control' women and men - random people stopped on university campuses to have their photos taken. Three of these pictures are chosen for you - that is, three pictures of women who are about the same age as you are. These photos are placed, along with your photo, on the right-hand side and, on the left-hand side, is a single picture of your boyfriend's mother. You can see where this is going...
Now, 120 people - ordinary people who were just stopped on the street - are asked to rate the faces on the right (that's the 4 faces, one of which is yours) in terms of how similar each of them is to the picture on the left (that's your boyfriend's mum). They do this on a scale from 1 (not at all similar) to 10 (extremely similar).
The bad news and the good news
Your photo is much more likely to be judged similar to your boyfriend's mother than the control women. To broaden that out, women in general were judged as much more similar to their boyfriend's mother than the photos of the control women. But the effect was not found the other way around: men were not judged to be be any more similar to their girlfriend's father than the control men.
What's going on?
There are a few explanations for this:
It could be a form of sexual 'imprinting' - the same basic process as baby ducks immediately following around the first maternal thing they see (because that helps them survive).
It could be that we prefer faces that remind us of, well ...us. Why? Again, there's an evolutionary angle here. A recent study found that marriages between third or fourth cousins in Iceland produce, on average, more children than those between completely unrelated individuals. The thing is, someone genetically similar to you is likely to be well-adapted to your environment - so it's more adaptive to find a partner similar to you than someone totally (genetically) different. Of course, you don't want TOO similar because that causes genetic problems so there's the 'Westermark' effect which stops you from finding immediate family attractive.
It could be the Electra or Oedipus complex - an unconscious desire for your opposite-sex parent still burning away from your first, toddler feelings of love/lust for them. The person who resembles your opposite-sex parent is basically the next best to the real thing.
A more contemporary (and more palatable) version of Electra/Oedipus is the idea of an Internal Working Model. This is your understanding of relationships, learned from the first, most intense relationships with your parents. You will have an internal working model that answers questions such as How worthy of affection am I? How much can I trust other people? And the (largely unconscious) answers to these questions will vary a lot depending on your early relationships with your parents.
The Psych Hack
The Psych Hack here is simply to be aware of these processes. Most often, it's fine to date people like our parents. But sometimes it isn't. Those who experience abuse as a child are much more likely to be victims of domestic abuse as adults. This has nothing to do with physical attraction, but it's hard to explain such findings without using ideas such as the Internal Working Model. So be aware - find partners who reflect all the best things you experienced as child. Reject and react against partners who mirror difficult, abusive experiences from your childhood.
Reference: Urszula M Marcinkowska & Markus J Rantala (2012) Sexual Imprinting on Facial Traits of Opposite-Sex Parents in Humans. Evolutionary Psychology. 2012. 10(3): 621-630