May 13, 2022 Uncategorized

The Facial Symmetry Hypothesis and The Sexual Selection


Module 6 Lab: Human mate choice

In many human cultures there tends to be agreement over what constitutes attractiveness – with people from different societies often agreeing that a given woman or man is ‘attractive’.  It is less clear, however, what actual traits observers use to make this judgement.  In this week’s lab, we are going to test a number of hypotheses to determine if there is a good way to subjectively measure attractiveness.


Sexual selection and good genes hypothesis:  Many species display the phenomenon of sexual selection – where males display costly ornaments or behaviors (although in some cases it is females that display these costly ornaments or behaviors).  According to parental investment theory, the gender that has lower intrinsic investment in reproduction is forced to compete for the attentions of the gender with the higher investment (Trivers, 1972).  There is still a fair amount of theoretical discussion concerning the costs and benefits of sexual selection.  One perspective, the ‘good genes’ concept, suggests that costly ornaments are essentially ‘honest signals’ that the bearer is carrying high quality genes.  An animal that is capable of maintaining a large feather display or complicated nest-building behavior must also be generally healthy and the carrier of desirable genetic qualities. 

Humans are not exempt from sexual selection theory.  Males and females are dimorphic in size and also in secondary sexual characteristics.  The human face contains secondary sexual traits that include facial features that may develop or increase in size at puberty.  Humans make judgements about the attractiveness of prospective partners based in part on these facial cues.  An attractive partner may elicit greater efforts at courtship or more costly displays to gain the attentions of the would-be mate.  But how do humans make these judgements about attractiveness?

Facial Symmetry Hypothesis:  One hypothesis regarding human attractiveness is facial symmetry.  Symmetry is often correlated with genetic heterozygosity in many animals, including humans (Thornhill and Gangestad, 1993).  Such heterozygosity may be linked to parasite resistance as well as other diverse positive traits and therefore symmetry may be an indicator of ‘good genes’.  There is evidence that facial symmetry is positively correlated with scores of attractiveness (Grammer & Thornhill, 1994; Fink et al. 2006), and it may also act as a cue to an individual’s personality characteristics (Fink et al., 2006). 

Averageness Hypothesis: Another hypothesis is the averageness hypothesis (Thornhill & Gangestad, 1993).  Based on this idea, researchers compiled photos of different people.  They found that the more people’s faces were averaged together, the higher the attractiveness score became. Thornhill and Gangestad (1993) suggest that facial averageness is attractive because of its association with heterozygosity.  It should be noted that the facial symmetry hypothesis and averageness hypothesis are not mutually exclusive.

Questions:  In order to investigate these questions, you will be utilizing an interactive website to rate human faces for attractiveness and also to create average faces (in which individual faces are blended together).


Access the website at:

To create averages, select faces you want to average and they will be show boxed in yellow.  To unselect an image, just click it again.

When you’ve selected the ones you want in your selection, click ‘average’.  Between trials, click the reset button to clear your previous selections.

Assignment instructions:

Assign attractiveness scores to the first 20 faces of each sex on a scale from 1 – 10.

Of the 20 faces, choose three women you assigned the lowest scores, and average their faces. Score the new composite face.

Of the 20 faces, choose three women with scores close to 5 and average their faces. Score the result

Of the 20 faces, choose three women with the highest scores and average their faces. Score the resulting face.

Average all 20 female faces. Score the resulting face.

Repeat steps 2-5, using male faces.

Here is a video example of me walking through the lab:

Report Instructions:  You will need to submit a formal lab report.  The report should be in a word document with the following sections:


An introduction containing a description of the basic questions and the premise (ie, what are the researchers testing with their facial averaging software?). Be sure to write and include both a description of each hypothesis (symmetry and averageness) as well as your research questions (Links to an external site.)… related to these.

Materials and Methods

In paragraph format and in past tense, describe the materials you needed and the methods you used to complete this lab. Similar to past modules, we understand this is an online lab so this section is a little difficult, but remember, someone (who is not in this class) should be able to complete the lab based on what you write here so what should you include?

  1. Results
  2. You should summarize your findings in a series of data tables AND graphs.  
  3. Submit a data table with your results summarized. Write a summary (in a word document) answering the following:
  4. Compare scores between the original faces and the average faces you created. How do they differ?
  5. Compare the scores for the composite faces across the low, medium, and high scoring categories.
  6. Does the score the average face change with how many individual faces are used to create it?

Submit a graph or graphs illustrating your data

For example, you might include a bar graph (or series of bar graphs) comparing the average attractiveness of the sets of three faces you selected (less attractive, medium attractiveness, and very attractive) before and after averaging.  


Finally, you should summarize your findings in your lab report.  

Was the ‘averageness’ hypothesis supported?

  • Was the ‘facial symmetry’ hypothesis supported?


Fink, B, et. al, (2006).  Facial symmetry and judgements of attractiveness, health, and personality.  Personality and Individual Differences 41: 491-499

Grammer, K, Thornhill, R (1994).  Heritable true fitness and bright birds: A role for parasites?  Science 218: 384-387

  • Thornhill, R, Gangestad, S (1993).  Human facial beauty: Averageness, symmetry and parasite resistance.  Human Nature 4: 237-269

Trivers, R. (1972).  Parental investment and sexual selection.  In B. Campbell (Ed.) Sexual selection and the descent of man, 1871-1971.  (pp 136-179).  Chicago. IL

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