The article by MacGregor Legge et al ^{Reference MacGregor Legge, Sendi, Cole, Cohen-Woods, Costafreda and Simmons1} focused on brain derived neurotrophic factor's (BDNF's) Val66Met polymorphism contribution to major depressive disorder and sought to determine whether the same neural effects would be observed in healthy individuals. There was a specific focus on cortical thickness in the amygdala, prefrontal regions of the anterior cingulate cortex and middle frontal and orbitofrontal cortices. The study focused on 79 patients with diagnosed major depressive disorder and 74 control participants, all of White European ancestry. The main effects were of Met carrier status on cortical thinning in the caudal middle frontal cortex in patients with major depressive disorder and controls. The polymorphism in the caudate middle frontal cortex was greater and no significant interaction was found in the amygdala.

One limitation not covered by the authors was the lack of ethnic diversity among the sample population. One previous study ^{Reference Ribeiro, Busnello, Cantor, Whelan, Whittaker and Deloukas2} investigated the BDNF polymorphism in an exclusively Mexican American experimental and control population and found it also to be associated with major depressive disorder. However, previous studies have found that the polymorphism has allele frequencies dependent on ethnic background. In White populations, the Val allele is found to be the most common and the frequency of the Met allele is 25% to 32%. ^{Reference Cargill, Altshuler, Ireland, Sklar, Ardlie and Patil3,Reference Shimizu, Hashimoto and Iyo4} In Asian populations the Met allele is more frequent, about 40% to 50%. ^{Reference Choi, Kang, Lim, Oh and Lee5–Reference Tsai, Cheng, Yu, Chen and Hong7} As MacGregor Legge et al found that the Met allele had the greatest effect on major depressive disorder, one would assume it would be of paramount importance to examine how differing frequencies affect the occurrence of the disorder. Pivac et al ^{Reference Pivac, Kim, Nediæ, Joo, Kozariæ-Kovaciæ and Hong8} in their study on ethnic differences in BDNF Val66Met polymorphism in a Croatian and Korean non-clinical sample found that polymorphisms and mood disorders may be dependent on ethnicity.

Searches for studies of the BDNF polymorphism in African–Carribbean and other ethnicities obtained no results. As generalisable conclusions cannot be drawn on the varying effects of the BDNF polymorphism on major depressive disorder or any major psychiatric illness in different ethnic groups, further examination into this topic would be significant.