Saturday, November 04, 2006

Why blue-eyed men prefer blue-eyed women - but not vice versa

Most people learn in high school (if they're paying attention) that human eye color is a genetic trait which follows fairly simple rules. The primary gene that controls eye color exists in several forms, called alleles. The protein produced by one of the alleles causes the eyes to be brown, while a variant allele producing a slightly different protein does not. Since the human genome contains two copies of each gene, if even one of these copies is the allele for brown eyes, brown will be the resulting color, regardless of the other allele. Such an allele is said to be "dominant". Another eye color (e. g. blue) will result only if both copies of the gene are non-brown alleles. Such alleles are said to be "recessive".

Children receive one copy of each paired chromosome from each parent. It follows that if one parent has two copies of a dominant gene, every one of their children will receive at least one copy, regardless of what the other parent has. All children of this mother and father will have brown eyes, if even one parent has two brown eye alleles, and even if the other parent has two blue eye alleles.

If both parents have one brown and one blue allele, then for any particular child, there's a 1 in 4 chance of receiving two brown alleles, a 1 in 4 chance of receiving two blue alleles (the only case that will result in blue eyes), and a 2 in 4 chance of receiving one brown and one blue allele (hence brown eyes). If one parent has blue eyes, and the other has both a brown and a blue allele, then the odds are 50/50 for each of their children to have either brown or blue eyes. So if one or both parents have brown eyes, it's possible for them to have blue-eyed children. But when both parents have blue eyes, so all of their alleles are for blue eyes, all of their children will have blue eyes. In that case, if any child has brown eyes, it must be the case that one parent – most likely the male – is not the biological parent. Oops.

So a blue-eyed man has an interesting advantage over men with brown eyes – a very dependable way of knowing that he is not the father of a particular child, provided he mates with a blue-eyed woman. Further, a blue-eyed man who regards blue-eyed women as more attractive than women of other eye colors is more likely to mate with blue-eyed women. And so such a blue-eyed man has a selective advantage over other blue-eyed men who have no such preference (or a preference for brown-eyed women).

This would be advantageous, at least in prehistoric times, if in addition such a man was less inclined to provide for a child without blue eyes – even if there was no conscious recognition that the child could not be his own. Some recent research has indicated that blue-eyed men sometimes actually, if unconsciously, do have a tendency to regard blue-eyed women as more "attractive", and hence (presumably) are more likely to choose them as mates:

Blue Eyes -- A Clue To Paternity
Eighty-eight male and female students were asked to rate facial attractiveness of models on a computer. The pictures were close-ups of young adult faces, unfamiliar to the participants. The eye color of each model was manipulated, so that for each model's face two versions were shown, one with the natural eye color (blue/brown) and another with the other color (brown/blue). The participants' own eye color was noted.


Since a mother almost always can be sure a given child is hers (except for rare events like accidental switching of infants), a mechanism that provides a way to recognize that a child isn't her own provides little additional advantage. And so, blue-eyed women do not have an evolutionary advantage from a tendency to regard blue-eyed men as more attractive than others. So they do not, in fact, have that tendency.

If you're looking around for an example of specific, and unexpected, behavior for which evolutionary psychology offers the simplest explanation, this may be a good choice.

Update 8/3/08: There is more recent news on this subject here and here.

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Anonymous Anonymous said...

Your information about eye color is almost but not absolutely correct. Though uncommon to rare, it IS possible for blue-eyed parents to have brown eyed children. None the less, though it happens, (and for a variety of reasons) it's rare and thus doesn't materially affect your story, nor your reasoning.

7/29/2008 10:49:00 AM  
Blogger Charles Daney said...

Your information about eye color is almost but not absolutely correct.

Thanks. There's actually some more recent information about this, which clarifies the genetic situation. I'll try to write about it soon - look for updates to this article for a pointer to new info.

7/30/2008 02:10:00 AM  
Anonymous Anonymous said...

That is weird. I never knew this before stumbling onto this page.

I am 25 and brown-eyed, and have had 5 serious, long-term relationships in which all but one of the men had blue eyes. In fact, the relationship with the brown-eyed man was the most unstable of the relationships. And I am currently married to a blue-eyed man.

Is it possible that the sample group, being in Norway, might have been influenced by their environment? The concentration of blue eyes in Scandinavia is very high, so perhaps their preference were more cultural?

While the logic behind the results of this study seems strong, I would like to see a larger sample before I take this hypothesis seriously. I would like to see a study done in more diverse cultures like the United Sates, Canada and/or the more diverse parts of Europe.

2/18/2010 11:01:00 PM  
Blogger Charles Daney said...

Yes, it will be interesting to see similar studies in other cultures. But since blues eyes are rather less common outside of Northern Europe - especially until very recently - it's possible that there are cultural prejudices against blue-eyed people, which might lead to different results.

See here for a study that shows blue eyes may have appeared only about 10,000 years ago.

2/19/2010 06:31:00 PM  
Anonymous Julie said...

To the above "anonymous" poster,

You express disbelief in these findings and provide your own anecdotal experience as empirical evidence. You are clearly not a scientist. You state having had five long term relationships and all but one had blue eyes, which are dissimilar to your own brown eyes. You are twent five years old...that is a fairly high frequency turn over of relationships, so I would suggest that your story could also be interpreted to be in favour of the current study's findings. It didn't work out between the brown eyed girl and the blue eyed boy...four times.
Just my thoughts, back to work now.

9/08/2010 04:21:00 PM  
Anonymous Anonymous said...

Hmm what a coincidence. Im south american of European origin and blue eyed. Blue eyes in my country are rarely seen and I have always seen blue eyed women more attractive. I didn't know why but i always planned on mating with a blue eyed female.

4/27/2012 09:56:00 PM  
Anonymous Jen said...

My daughter has hazel eyes (she is 25) and her father and I both have blue eyes. His are baby blue and mine are more of a blue gray. He is the biological father. I wasn't sexually involved with anyone else. She looks just like him and I was conscious during her birth. It is unusual but I have all the empirical evidence needed. I hope this helps!

6/23/2012 05:19:00 AM  
Blogger Charles Daney said...

The genetics of eye color are complicated. Two people with blue eyes can (obviously) have a child with eyes of another color. It's possible, but fairly improbable. What happens in evolution is all based on probabilities.

6/23/2012 12:17:00 PM  

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