What is the D2 index finger?

The "D2 index finger" refers to the second finger on the hand, commonly known as the index finger. In scientific and research contexts, fingers are often labeled as D1 through D5, starting from the thumb (D1) to the pinky (D5). The index finger, therefore, is D2. This terminology is used primarily in studies and discussions about digit ratios, particularly the D2 to D4 ratio. This ratio has been studied for its potential association with various biological and behavioral traits, such as hormonal exposure in utero 1.