Caterpillar
infected by baculovirus, before liquefaction (Wikicommons, Williams et al.
2017)
Higher
IQ is associated with antibodies to cytomegalovirus, but only in adults whose
IQ is already above a certain level. Which is the cause, and which is the effect?
Cytomegalovirus
(CMV) belongs to the herpes family and is spread by contact with bodily fluids.
Around 45 to 100% of the population may be infected, although most hosts are
unaware that they are infected. It has one of the largest genomes among human
viruses, a sign perhaps of its ability to manipulate its host:
With millions of years of coevolution
within their hosts, CMVs, like other herpesviruses, encode numerous proteins
that can broadly influence the magnitude and quality of both innate and
adaptive immune responses. These viral proteins include both homologues of host
proteins, such as MHC class I or chemokine homologues, and proteins with little
similarity to any other known proteins, such as the chemokine binding protein.
Although a strong immune response is launched against CMV, these virally
encoded proteins can interfere with the host's ability to efficiently recognize
and clear virus, while others induce or alter specific immune responses to
benefit viral replication or spread within the host. (Miller-Kittreall and
Sparer 2009)
CMV
infection at birth leads to mental retardation (Andreou et al. 2021). Recently,
however, it has been shown that infection later in life can lead to higher IQ.
This is one of three findings from a Norwegian study on IQ and antibodies to
CMV in adults. The study showed that CMV seropositivity was significantly
associated with higher IQ in men who suffered from bipolar spectrum disorders
and with lower IQ in women who suffered from schizophrenia spectrum disorders.
CMV seropositivity was not associated with IQ in healthy controls (Andreou et
al. 2021). The study’s authors were at a loss to explain the association
between CMV seropositivity and higher IQ in bipolar men, all the more so
because the number of bipolar men was small, only 35.
An
association between CMV and IQ has now been found in healthy individuals. A
Czech study has shown that IQ, especially verbal IQ, is higher in people with
antibodies to CMV. Moreover, the IQ advantage decreases with decreasing levels
of CMV antibodies, i.e., with increasing time since the CMV infection
(Chvatálova et al. 2022).
Why
do healthy Czechs show this association but not healthy Norwegians? It’s not
because the Czechs were a larger sample. In fact, the Czech sample had 283
healthy individuals, and the Norwegian sample 474. The two samples did differ,
however, in educational attainment. The Czechs were biology students at a
university in Prague, whereas the Norwegians were randomly recruited from the
Norwegian population register. The latter were also described as “Caucasians”
living in Oslo. Oslo’s population is almost one third of immigrant origin, with
Pakistanis forming the largest immigrant group. There are also large numbers of
people from Sri Lanka, Turkey, Morocco, and Iraq (Wikipedia 2022).
The
last point would not be problematic if cognitive evolution had ended long ago
among the common ancestors of Europeans, Middle Easterners, and South Asians.
There is mounting evidence, however, for cognitive evolution in recent times.
Ashkenazi Jews seem to have gained their cognitive advantage during the past
1,000 or so years, and the same seems to be true for the Parsis (Cochran et al.
2006; Dunkel et al. 2019; Frost 2021). There is even evidence for significant
cognitive evolution in communities that are not normally thought of as ethnic
groups, such as French Canadians in regions where British and American traders
were historically few in number (Frost 2012).
Why does CMV
seropositivity correlate with higher IQ?
The
authors of the Czech study suggest that more intelligent people have more
social contacts and are thus more likely to catch the virus: “we suggest that
more intelligent subjects who have more social and sexual contacts—CMV is
transmitted by close contacts, e.g. by kissing—might have a higher risk of
encountering a CMV infection.” Yet there is no evidence that smarter people are
more extraverted. In fact, they tend to be loners, if only because they have
fewer people of their intellectual level to hang out with. The academic
consensus seems to be that neither introversion nor extraversion correlates
with intelligence (Saklofske and Kostura 1990).
Could
the arrow of causality run in the other direction? Is it possible that CMV makes
its host smarter? The authors reject that explanation because congenital CMV
infection reduces intelligence. But maybe the effect is different in adults.
If
CMV does increase the intelligence of adult hosts, the effect would be confined
to those whose IQ is already above a certain level. The above two studies
showed a significant increase only among university students, and not in a more
mixed population with varying levels of educational attainment. But why did the
latter study show significantly higher IQ in men with bipolar disorders and
significantly lower IQ in women with schizophrenia? For the answer, we can turn
to the results of a recent genome-wide association study: most of the alleles
for schizophrenia are associated with lower intelligence, and most of the alleles
for bipolar disorder are associated with higher intelligence (Smeland et al.
2020). Those results are partially confirmed by the findings of a prospective
cohort study: “at least in men, high intelligence may indeed be a risk factor
for bipolar disorder, but only in the minority of cases who have the disorder
in a pure form with no psychiatric comorbidity” (Gale et al. 2013).
Behavior
alteration?
Why
would a virus try to make its host smarter? What would it gain? Perhaps the
increase in intelligence is a side-effect. Perhaps the virus is trying to
improve its chances of spreading to new hosts by altering the behavior of its
current host (Cochran et al. 2000; Frost 2020).
Although
behavior is more often altered by larger and more complex pathogens,
particularly fungi, there are many viruses that engage in behavior alteration.
For example, viruses from the baculoviridae
family will infect a caterpillar and make it hyperactive to spread their viral
progeny over a wider area. Or the caterpillar will be made to climb to the top
of a plant and dissolve itself through overproduction of enzymes, thus becoming
a mass of tasty goo for ingestion by potential hosts (Han et al. 2015; Williams
et al. 2017). Rabies is another behavior-altering virus: it makes its host more
aggressive and thus more likely to bite potential hosts.
Although CMV infects a wide range of people, it seems to target a smaller subgroup for behavior alteration, i.e., individuals with intelligence above a certain threshold, and men more than women. If we look at the epidemiological data, we see that male homosexuals are especially susceptible. A study at a venereal disease clinic found that antibodies to CMV were present in 94% of the male homosexual patients and 54% of the male heterosexual patients. “The data suggest that sexual transmission is an important mode of spread of CMV among adults and that homosexual men are at greater risk for CMV infections than are heterosexual men” (Drew et al. 1981). Another study has identified passive anal sex as the most effective means of transmission: “Of seven sexual practices investigated, only passive anal-genital intercourse correlated with the acquisition of cytomegalovirus infection (p =0.008)” (Mintz et al. 1983).
Which
is the cause and which is the effect? Does passive anal sex facilitate CMV
infection? Or does CMV infection facilitate the desire for passive anal sex?
The answer may be ‘yes’ to both questions. Sometimes ‘the cause’ and ‘the
effect’ are two sides of the same coin.
This
virus may indeed be the ‘gay germ’ that Greg Cochran has written about. Or one
of them. Male homosexuality probably has several causes, and the microbial
cause probably involves more than one pathogen.
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