The olive-sized amygdala is a key brain structure involved in all
types of emotion and in social behaviors such as aggression and sexual
arousal. Animal studies and early MRI reports indicated that the
amygdala is disproportionately larger in males' brains.
Such a size
difference has been suggested to contribute to sex differences in
emotionality and in the prevalence of disorders such as anxiety and
‘Despite the common impression that men and women are profoundly different, large analyses of brain measures are finding far more similarity than difference.’
Despite the common impression that men and women are profoundly
different, large analyses of brain measures are finding far more
similarity than difference.
The latest evidence to address this controversy comes from a study at Rosalind Franklin University of Medicine and Science, where a
meta-analysis of human amygdala volumes found no significant difference
between the sexes. Meta-analysis is a statistical approach for combining
the results of multiple studies, in this case dozens of brain MRI
Lise Eliot, principal
investigator and associate professor of neuroscience at RFU's Chicago
Medical School. "There is no categorically 'male brain' or 'female
brain,' and much more overlap than difference between genders for nearly
all brain measures."
Biologists use the term "sexually dimorphic" (literally, "two
different forms") to describe male-female differences. This new study
shows that the term does not apply to human amygdala volume. It joins
other recent research that challenges the concept of binary "male" and
"female" human brains, and may have relevance to understanding disorders
including depression, substance abuse, and gender dysphoria.
The report is co-authored by RFU medical students Dhruv Marwha and
Meha Halari, who worked with Eliot to systematically identify all MRI
studies of the human amygdala over the past 30 years. They found 58
published comparisons of amygdala volume in matched groups of healthy
men and women (or boys and girls) that included 6,726 total
Studies reporting raw amygdala volume show that the
structure is indeed about 10% larger in male brains. However,
this difference is comparable to males' larger body size, including the
11-12% larger volume of males' brains overall. Among studies that
reported amygdala volumes corrected for overall brain size, the volume
difference was negligible (<0.1% in the right amygdala, 2.5% in the left amygdala) and not statistically significant.
The paper appears in the journal NeuroImage
In a similar 2015 meta-analysis,
Dr. Eliot headed a Chicago Medical School team that debunked the
widely-held belief that the brain's hippocampus, which consolidates new
memories, is larger in women than men.
"There are behavioral reasons to suspect a sex difference in the
amygdala," Dr. Eliot said. "Emotion, empathy, aggression, and sexual
arousal all depend on it. Also, the evidence from animal studies
suggesting a sex difference in amygdala volume is stronger than it is
for the hippocampus. So this finding is more surprising than our
hippocampal result and suggests that human brains are not as sexually
dimorphic as rats."
This study strengthens the case for gender similarity in the human brain and psychological abilities and has implications for efforts to understand the transgender brain.