The Scent of Symmetry: A Human Sex Pheremone that Signals ...

The Scent of Symmetry: A Human Sex
Pheremone that Signals Fitness?
Thornhill and Gangestad, 1999
Presented by Jon Kho

Defining Symmetry/Asymmetry
• Symmetry
– Based upon traits that are, on average, bilaterally symmetrical
• Fluctuating Asymmetry (FA)
– An average of any symmetrical deviation
– Low FA = “Good”
– High FA = “Bad”
Key Reference: A study across 42 different species
- On average, low FA individuals mate more
- Low FA individuals found more attractive

Overview of Study #1
• Only looked at females ratings
• Smaller sample
• Confounds not addressed
Revised study maintained format while
addressing these issues

• 82 Females
– 17-53 years of age
Subjects
– No use of oral contraception
• Some did and this was addressed
• 80 Males
– 17-33 years of age
• Portrait taken
Ethnic majority were Caucasian, though in prior study, ethnicity
was deemed irrelevant to perceived attraction

Symmetry Measurement
• 10 Key traits measured within 0.01mm
– Ear Length
– Ear Width
– Elbow Width
– Wrist Width
– Ankle Width
– Foot Breadth
– Lengths of all fingers except the thumb
• FA index
– Relative FA (per individual)
– Absolute FA (compared to sample mean)
Measurements taken and questionnaire filled

• Injury
• Rule Violations
– Showers
– Scented hygiene products
Confounds

• Unworn Hanes cotton shirt
• Explicit Instructions
– Worn 2 nights during sleep
The Shirt
– Wash sheets with provided detergent
– Refrain from scented hygiene products (soap provided)
– Refrain from notorious foods
– No drugs, alcohol or tobacco
– No sex and no bed mates
Dropped shirts off and filled out questionnaire noting any violations

Smell Ratings
• Split into groups of ~5
• 10 shirts per group, with one control added
• Smell shirt then rate on:
– Pleasantness
• 1(very unpleasant) – 10(very pleasant)
– Sexiness
• 1(very unsexy) – 10(very sexy)
– Intensity
• 1(not at all intense) – 10(very intense)
• Double Blind setup
• Women asked questions regarding menstrual cycle
• 14 female shirts removed from analysis

Facial Attractiveness
• 14 women and 15 men
• 1-10 rating of attractiveness
• High internal consistency

Fertility Groups
• 2 methods averaged for Fertility Risk score
– Last period compared to a table
– Last period in relevance to self reported cycle
• High Fertility group
– 13 women (out of 48 eligible women)
– .16-.39
• Low Fertility group
– 35 women
– .01-.11

Results

• Intensity Ratings
Results
– No significant difference between High/Low Fertility groups
• Facial Attractiveness
– Correlation between facial attractiveness and High Fertility
• High Fertility rating less reliable due to small sample size
– Facial Attractiveness and FA were not correlated

• High Fertility Group
Results
– Negative relationship between FA and attractiveness ratings
– Relationship was significant when confounds were removed
• Low Fertility Group
– No significant relationship between FA and ratings
• Men
– Low FA males tended to favor more symmetrical women
• Small correlation

Discussion
• Possible increased sensitivity to pheromone during peak
fertility windows
• Results were independent of age, socio-economic
status, or weight

Problems
• High Fertility group was small
• Conclusions were merely statistics
• Connection between Pleasantness and Attractiveness?