July 5, 2011 -- The ratio between the length of the index finger and the ring finger of men's hands is associated with penis length, a study shows.
Researchers found that men with a lower ratio, or a shorter length of the index finger compared to the ring finger, tended to have a longer penile length.
The study is published in the Asian Journal of Andrology.
In earlier research, the ratio, known as the digit ratio, has been linked to sexual behavior or hormonal activity. A recent study linked digit ratio to facial attractiveness or "hotness," and another found an association between digit ratio and prostate cancer risk.
Those findings have prompted some to suggest that digit ratio may serve as a marker for prenatal androgen or testosterone exposure.
"Based on this evidence we suggest that the digit ratio can predict adult penile size and that the effects of prenatal testosterone may in part explain the differences in penile length," write study researcher In Ho Choi of Gachon University Hospital in Incheon, South Korea, and colleagues.
In the study, researchers analyzed the relationship between digit ratio and penis size in 144 Korean men 20 or older who were hospitalized for urological surgery.
The length of the index and ring fingers of the right hand were measured prior to surgery, and flaccid and stretched penile lengths were measured while the participant was under anesthesia.
The results showed that after accounting for age, weight, height, digit ratio, individual finger length, and body mass index (BMI), only height and digit ratio were significantly associated with penis length.
However, height was associated with flaccid penile length only. Stretched penile length was not associated with height but was negatively associated with digit ratio. That is, those with a lower digit ratio tended to have longer stretched penile length.
Researchers say because digit ratio has been shown to vary among different ethnic groups, a strength of this study was that it was conducted in a single ethnic group of men.
Experts say these results may offer researchers insights beyond the obvious cocktail conversation.
"Over the past decade, the correlation of digit ratio with sexual behavior and other aspects of sexual biology has been well-documented and there is a growing list of traits with links to digit ratio, although the associations are less well established," writes Denise Brooks McQuade, of Skidmore College in Saratoga Springs, N.Y., in an editorial that accompanies the study.
"Thus 'hotness' aside, the value of digit ratio research for the biomedical scientist or clinician may come from the predictive abilities and risk-assessment qualities of the measurement," writes McQuade.