The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd (index finger) and 4th (ring finger), is affected by exposure to androgens. 

In 1983 Dr Glenn Wilson of King’s College, London published a study examining the correlation between assertiveness in women and their digit ratio. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero. In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children and further developed the idea that the index was a marker of prenatal sex hormones.

2D:4D ratios vary greatly between different ethnic groups. This variation is far larger than the differences between sexes

Some authors suggest that digit ratio correlates with health, behavior, and even sexuality in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with digit ratio.

Physiology and disease

• Sperm counts
• Heart disease
• Obesity & Metabolic syndrome
• Prostate cancer

Psychological disorders

• Autism
• Depression
• Schizophrenia
• ADHD
• Risk for eating disorders in females and males

Physical and competitive behavior

• Skiing
• Soccer playing
• Sporting in females
• Financial trading

Cognition and personality

• Assertiveness in women
• Aggression
• Masculinity of Handwriting
• Perceived ‘dominance’ and masculinity of man’s face
• Personality
• Exam scores: a higher ratio is correlated with higher exam scores among male students
• Musical ability