BDNF and psychiatric disorder: Many studies have suggested the BDNF Val66Met homozygous Met/Met allele (A/A) or heterozygous Val/Met alleles (G/A) were associated with a reduced risks of certain psychiatric disorders such as substance abuse and anxiety‐related personality traits (Gratacòs et al., 2007; Lang et al., 2005; Sen et al., 2003), although other studies with other conditions show different relationships (Gratacòs et al., 2007).