MRI-assessed volume of left and right hippocampi in females correlates with the relative length of the second and fourth fingers (the 2D:4D ratio).

Atrophy of the left or right side of the hippocampus has been related to cognitive deficits and psychiatric disease. In this study, we examined the correlation between the hippocampal volume laterality index and the relative lengths of the second (index finger) and fourth (ring finger) digits (2D:4D) in healthy female subjects. The 2D:4D ratio is fixed in utero, and the ratio is higher in women than in men. There is evidence that this ratio is an indicator of the intrauterine concentration of testosterone, which influences the development of different regions of the brain. Assessing the volume of different parts of the brain of 40 healthy adult female students by magnetic resonance imaging (MRI), we found that the 2D:4D ratio was associated with an asymmetry in the hippocampal sub-regions. Smaller volume on the left side was found in the posterior part of the hippocampus in females with a low (masculine type) 2D:4D ratio. On the other hand, smaller volume on the left side was found in the middle part of the hippocampus in females with a high (female type) 2D:4D ratio. Thus, the development of the middle and posterior regions of the hippocampal formation may respond in opposite ways to prenatal levels of testosterone. Other brain regions such as the amygdala, the cerebral cortex, the total volume hippocampus, and the head of the hippocampus did not show such a difference.