44 | New Scientist | 12 October 2019
compounds by microbes living on our bodies,
says Natsch. We know genetics plays some part,
and that it is the microbes, not the precursors,
that determine how ripe you get. But we still
don’t know all the steps in the process.Getting sniffy
In general, people with lots of bacteria in
the genus Corynebacterium, which are more
common in men, tend to smell pungent,
whereas those with lots of Staphylococcus
are less smelly. Men also tend to sweat more,
releasing more precursors to feed those
musky microbes – another reason they are
generally the stinkier sex.
It may come across as rude, but recoiling
from someone’s oniony underarms is
a long-evolved reaction. “There’s a high
sensitivity of the human nose to these
odorants,” says Natsch. “This indicates that
they had some function in human history,
even if today they are just malodours.”
Until recently, it was thought that we could
detect only about 10,000 scents. In 2017, this
idea was exposed as a myth by John McGann at
Rutgers University in New Jersey. Comparing
the neuroanatomy of 24 mammals, he found
that humans can smell up to a trillion different
odours – similar to a dog or a rat.
Still, the notion that smell is less important
for us persists. “We humans don’t typically
walk up to strangers and overtly sniff them,”
says Noam Sobel at the Weizmann Institute
of Science, Israel. “Or do we?” His team has
shown that we sniff our hands a lot and are
twice as likely to do so after shaking handswith a stranger. Handshaking is just one of
the subtle ways we sample someone else’s
scent. “People constantly sniff themselves
and others,” says Sobel.
What are we trying to suss out with all of this
sniffing? Despite popular enthusiasm for the
idea, there is still no good evidence for specific
pheromones that drive sexual attraction (see
“Love potion”, opposite). But many researchers,
including Tristram Wyatt at the University
of Oxford, think we probably do produce
pheromones – chemicals that have evolved to
trigger a specific behaviour in the individual
who smells them. He believes the first will be
discovered between mothers and their babies,
where we already know scent plays a role
in bonding and that mothers can recognise
their infant’s smell minutes after birth.Identical twins Chris and Xand Van Tulleken talk about
how to beat your genes at New Scientist Live on 13 October
newscientistlive.com“ Handshaking
is just one of the
subtle ways we
sample someone
else’s scent. We
constantly sniff
each other”
Body odour undoubtedly plays a broader
role in kin recognition, but studies show
surprisingly inconsistent results. “Kin
recognition is based on both familiarity and
a genetic component,” says Ilona Croy at the
Technical University of Dresden, Germany. She
notes that not all mothers recognise the smell
of their children but, if they do, they much
prefer it to the smell of other kids. “Familiarity
seems to be more important,” she says.
The genetic component of kin recognition
by smell is puzzling. It is thought to rest on
the human leukocyte antigen (HLA) set of
genes, which varies a lot between individuals
and codes for proteins associated with
immunity. It has been assumed that humans
are sexually attracted to the smell of people
whose HLA is dissimilar to our own, based on
reams of research suggesting this is the case for
many animals. That would be an evolutionary
boon for offspring because the more diverse
a person’s HLA, the stronger their immune
system. But studies in humans have been
disparate and controversial, and a 2017 meta-
analysis failed to confirm that HLA drives
sexual attraction, even though it did show that
we prefer people with high HLA diversity. “This
is just a small part of partner choice,” says Jan
Havlíček at Charles University, Czech Republic,
whose own studies reveal that the HLAs of real
couples are actually more similar than those of
randomly chosen ones.
Beyond bonding, our bodily odours convey
a whole other world of meaning, particularly
where emotions are concerned. For example,
a meta-study published in 2017 leaves no
doubt that we can smell fear, stress and