SIZE AT BIRTH AFFECTS MENTAL DISORDER RISK
New research from the
Copenhagen Centre for Social Evolution and Yale University offers compelling
support for the general evolutionary theory that birth weight and -length can
partially predict the likelihood of being diagnosed with mental health
disorders such as autism and schizophrenia later in life. The study analyzed
medical records of 1.75 million Danish births, and subsequent hospital
diagnoses for up to 30 years, and adjusted for almost all other known risk
factors. The study is published in theProceedings of the
Royal Society, London B.
The number of people
diagnosed with mental health disorders is on the rise in most affluent
countries, but we do not yet have a comprehensive understanding of the factors
that make people vulnerable to these disorders.
A new analysis of the
extensive Danish public health database suggests that part of the answer may
reside in genetic imprints established at conception that influence both size
at birth and mental health during childhood and early adolescence.
The study tests
predictions of the evolutionary theory of genomic imprinting -- the idea that
during fetal development some genes inherited from the mother are expressed
differently to those inherited from the father. The potential consequence of
this asymmetry is that maternal and paternal genes in a fetus will not
cooperate fully during this period, even though they subsequently have shared
interests due to their lifetime commitment to the same body.
Opposite forces
balance each other
The reason for the
conflict is that some of the genes known to be expressed in the placenta and
the brain carry imprints that affect resource provisioning of the unborn child.
When such genes come from the father, they favor investment of more of the
mother's resources in the developing fetus, whereas the maternally-imprinted
genes will normally compensate for such paternally-influenced manipulative
effects to lessen the drain on maternal resources. These opposite forces
balance each other in most pregnancies, with the result that most children are
born with close to average length and weight and with a high likelihood of
balanced mental health development.
Small deviations may
well be favorable in human populations, when somewhat heavier babies are more
likely to develop abstract talents and somewhat lighter babies above average
social talents, for instance. However, this incurs the risk of increasing the
frequency of autistic- and schizophrenic-spectrum disorders in the rare cases
where imprinting imbalances are larger. The theory may explain why natural
selection has not removed this portion of the burden of mental disease from our
ancestors.
The new study tests
these predictions, and its results are remarkably consistent.
They show that the change to the risk of developing mental disorders when born
smaller or larger than average are relatively small, but very consistent,
clearly diametrical, and part of the single continuum that the theory predicts.
"When we started
this large scale analysis four years ago, we hoped to find evidence that
genetic imprinting happens, but we did not expect that the results would match
the predictions as consistently as we found," explains Professor Jacobus
Boomsma, Director of the Centre for Social Evolution, University of Copenhagen,
who coordinated the work.
Boomsma adds:
"Our study confirms that larger babies have a higher risk for incurring
autism-spectrum diagnoses later in life and lower risk for
schizophrenia-spectrum disorders. For example, Danish newborns are on average
52 cm long and being born at 54 cm increases the autism risk by 20%. However,
these are relative risks and these disorders remain rare: in this example the
absolute risk increases from 0.65% to 0.78%. Risk patterns are opposite in
smaller newborns, who have higher risks for schizophrenia and lower risks for
autism. Only for the smallest, prematurely-born babies does this diametric
pattern disappear, because they have elevated risks for almost all disease
categories."
Evolutionary conflicts
Boomsma also
underlines that focused genomic studies will be needed to find out which genes
are involved and how they affect brain function: "Our Centre's main
objective is to develop and test evolutionary theory about the ways in which
gene-level conflicts can corrupt even the most sophisticated forms of naturally
evolved cooperation. It is no surprise that humans are vulnerable to such deep
evolutionary conflicts, as are other mammals, and it is both useful and
interesting to be aware of this part of our biological heritage," says
Professor Boomsma.
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