What causes some people to be left- handed? Is handedness a uniquely human trait? Are left-handers more likely than other people to be creative geniuses or to suffer cogni- tive disabilities? Historian Howard
Kushner raises these and other questions in
his new book, On the Other Hand.
Primarily a history of handedness research,
the book also has an agenda: to show that sci-
entific conclusions have been influenced by
social attitudes about handedness.
Indeed, researchers’ personal views
can skew their interpretation of scientific data, a point made clearly in
Kushner’s book, although not in the
way that the author intended.
Today, being a lefty is a point of
pride in the Western world. Historically, however, the left hand has
been associated with atavism or
corruption and its use has been discouraged, if not savagely punished.
Kushner reports that in the early
1900s, for example, Zulu children
who could not remember to eat with
their right hand would have their left
hand immersed in boiling water. Less
extreme measures to “retrain” natural left-handers were common in the
United States as recently as the 1940s
and are still practiced in parts of India, Africa, and Asia.
Over several chapters, Kushner explores the biological underpinnings of handedness. He convincingly debunks the myth,
still believed by many neuroscientists, that
left-handers tend to have language functions
localized in the right cerebral hemisphere
and traces a century of efforts to discover
the origin of left-handedness. Kushner notes
that researchers’ tendency to pursue either
genetic or environmental causes for handedness reflects broader trends toward explaining human behavior in terms of nature or
nurture, which vary from one era to the next.
His clear-eyed view of large scientific lit-
eratures becomes clouded, however, when
Kushner turns from questions about brains
A history of handedness shows how attitudes can
influence scientific conclusions
By Daniel Casasanto
The reviewer is at the Department of Human
Development, Cornell University, Ithaca, NY 14853, USA.
and genes to more socially resonant questions. Despite his focus on handedness and
mental disorder, he does not engage with
studies showing that left-handers are more
likely to experience depression, the most
common mental disorder in the United
States (1). And in the final chapter, he tries
hard to discredit studies linking handedness with schizophrenia and autism—to the
To evaluate the evidence for these links,
Kushner draws on two recent large-scale
meta-analyses, one for each disorder (2, 3).
His primary criticism of both studies is that
they show statistical associations between
a mental disorder and non–
right-handedness, as opposed to left-handedness, per
se. He argues that comparing disease rates
in right-handers versus non–right-handers
constitutes “sleight of hand”: a “tactic”
that researchers use to provide a “rationale
for publication” of otherwise unpublishable results.
Crucially, Kushner claims that “none of
the studies, including those discussed [in
his book], has been able to demonstrate
that left-handedness alone is associated
with either schizophrenia [or] autism.”
He concludes that using “the often vague
category of non-right-handedness” has
“enabled research into the supposed pa-
thology of left-handers to continue, when
a focus exclusively on left-handers would
have eliminated this line of research.”
There are serious problems with this
argument. Most broadly, there is nothing
inherently wrong with the category “non–
right-hander,” which can be defined with
exactly the same level of precision (or impre-
cision) as the category “right-hander.”
Most problematically, Kushner is factually
wrong about the data on which his argument
rests, at least in the case of autism. In the
meta-analysis that Kushner condemns, the
researchers did not analyze left-handers and
mixed-handers separately, but they did pro-
vide all of the data needed to do so.
Using the same statistical test used in
the study, I performed this analysis,
which showed that the rate of left-
handedness was, in fact, significantly
greater in the autistic sample than in
the sample of healthy controls.
Kushner appears to be motivated
by the desire to conclude that left-handed people are actually normal
and that left-handedness is not a
pathology. But even if left-handers
are statistically more likely to develop autism or other disorders,
this doesn’t mean that left-handedness is pathological. It is well established that males are more likely
to develop autism than females, yet
maleness is not generally considered to be a pathology.
Kushner succeeds in showing that
left-handedness has been stigmatized and suppressed, but he fails
to show that all links between handedness
and disease are spurious or that researchers’
conclusions are based on “cultural norms”
about left-handers’ “abnormality.” By denying demonstrated links between handedness
and mental disorders, Kushner may hope to
prevent scientific findings from being used to
justify prejudice. But the problem is not with
the science, it’s with how people might use
it. And as Kushner’s engaging history illustrates, prejudice against the “sinister” hand is
rarely based on science. j
1. K.Denny, Laterality: Asymmetries Body Brain Cognition 14,
2. I. Sommeretal., Brit.J.Psychiatr.178, 344 (2001).
3. A.L.Rysstad, A.V.Pedersen, J. Autism Dev. Disord. 46,
Kushner claims that links between left-handedness and mental illness
are spurious, but are they?
On the Other Hand
Left Hand, Right Brain,
Mental Disorder, and History
Howard I. Kushner
Johns Hopkins University
Press, 2017. 216 pp.
Sleight of hand
INSIGHTS | BOOKS