Manning and Peters (2009) found in a large internet-based study surveying 255,116 participants that the ratio between second and fourth digit (2D:4D ratio), an index that

has been shown to be associated with testosterone levels in the womb and not associated with adult levels (Honekopp et al. 2007), is also associated with handedness such that lower 2D:4D ratios, indicating high prenatal testosterone, are associated with both left handedness and mixed handedness. Low 2D:4D ratios have also been shown to be related to low grehlin levels, a hormone implicated in hunger modulation (Jurimae et al. 2008), obesity, ADHD (Stevenson et al. 2007), Inhibitors,research,lifescience,medical aggressiveness and sensation seeking (Hampson et al. 2008), and selleck chemicals maternal smoking during pregnancy (Rizwan et al. 2007). Thus, testosterone exposure in utero may predispose to both left–right handedness biases as well Inhibitors,research,lifescience,medical as to other physiological differences with potentially long-lasting effects. Although this hormonal influence appears to be environmental, evidence showing that the 2D:4D ratio is substantially heritable is also available (Voracek 2008) and suggests that 80% of the variance in 2D:4D ratio is genetically determined (Medland and Loehlin 2008). Of particular relevance for the present study Kallai et

al. (2005) found that relative Inhibitors,research,lifescience,medical size of the left and right hippocampi was correlated with 2D:4D ratio in young healthy women. The possibility that early and lifelong hormonal levels modulate

neurodegenerative processes is somewhat supported by the present results showing greater hippocampal atrophy in males Inhibitors,research,lifescience,medical than in females. The possibility that early traumatic injuries of the central nervous system might predispose to left handedness Inhibitors,research,lifescience,medical or mixed handedness also finds some support in the literature. For instance, a twin study showed that the sibling with lower birth weight was at higher risk of lower IQ but only in the context of left handedness, suggesting an association between prenatal pathological events and handedness (Segal 1989). A recent prospective study following 1714 children before and after birth found that mothers’ depressive symptoms and critical life events (including interpersonal loss, financial difficulties, illness, or injury) before birth were MycoClean Mycoplasma Removal Kit associated with mixed handedness, which in turn was associated with a higher risk of language difficulties and ADHD symptoms at age 5 (Rodriguez and Waldenstrom 2008). Overall, the available evidence suggests that left handedness but particularly mixed handedness is associated with a number of risk factors, most of which have a strong genetic origin, which could lead to smaller hippocampal and amygdalar volumes and greater atrophy in ageing.