91av

TIME was, fingers were for playing the piano and pulling a trigger. The most
a set of hands could betray was how often the dishes got washed. Then scientists
announced that our fingers may reveal personal details about our sex
lives—whether we’re gay, for instance, and how likely we are to be
infertile—and suddenly hands are being slid deep into pockets. After all,
that’s not the kind of information you want to be waving around in public.

Sexual preferences and fertility aren’t the only secrets on display, either.
Scientists now suspect that fingers may reveal our risk of suffering a heart
attack or developing breast cancer. They may shed light on disorders such as
autism and dyslexia or gifts such as musical ability.

At first blush, the idea that a person’s hands can tell intimate stories
about them seems outrageous. But on closer inspection, there might be something
in it. Fingers are formed early in fetal development, at around the end of the
first trimester—a very critical time. Just as hand development is
influenced by the unique hormonal bath in the uterus, so is the development of
the brain, gonads and heart. Since the relative lengths of our fingers seem to
stay fixed throughout our lives, they are robust markers of what our early life
was like in the womb—fetal fossils, if you will.

Take a close look at your own hands. Pay particular attention to your index
and ring fingers. In women, the two fingers tend to be almost equal in length,
as measured from the crease nearest the palm to the fingertip. In men, the ring
finger tends to be much longer. The ratio of the lengths of the index finger to
the ring finger is called the 2D:4D ratio, and low ratios are considered
“masculine”, high ratios “feminine”.

Bodily characteristics that develop in distinctly masculine and feminine ways
are usually the product of sex hormones. Some features differentiate at puberty,
such as breasts and jaws. But other sex differences are already set by the time
we’re born, relative finger lengths among them, and seem to be the result of
fetal androgens masculinising the males. Some of those hormones come from fetal
testes and adrenals, the rest make it across the placenta from the mother. But
exactly how much comes from whom—and what alters the balance—are
still a bit of a mystery.

“Prenatal development is a black box,” says John Manning of the University of
Liverpool. He is one of a small number of scientists beginning to wonder if
fingers could be used as a way of peering into that box.

In a paper just published in the journal Medical Hypotheses (vol 54,
p 855), Manning highlights conditions such as heart disease, breast cancer,
autism and dyslexia. Both heart disease (in men) and breast cancer have been
linked with high levels of the female hormones oestrogen and progesterone. Most
of the studies of this link have looked at circulating levels in the adult, but
evidence is mounting that too much of the wrong hormone in the womb may be the
real culprit.

Early warning

Dimitrios Trichopoulos, an epidemiologist at Harvard University, proposed a
decade ago that breast cancer may originate in utero (The Lancet, vol
335, p 939). He suggested that high concentrations of oestrogen may create a
“fertile soil” for cancer to develop later in life. He also thought that
variability in oestrogen levels during pregnancy may help to explain why breast
cancer rates are generally higher in women born to Caucasian mothers compared
with those born to Oriental or younger mothers. Recently he and his colleague
Karin Michels showed that high birthweight in girls—another sign of high
prenatal oestrogen levels—was associated with an increased risk of breast
cancer.

If high oestrogen levels are indeed to blame, Manning thinks that high 2D:4D
ratios could be used to identify women who are at increased risk of breast
cancer. “I don’t know of other sexually dimorphic traits that are so stable,” he
says. “That’s what makes it so exciting.” He interviewed 118 women in a breast
cancer clinic, measured their finger lengths and noted how old they were when
the first tumour appeared. “It was earlier if there was a higher ratio,” he
says.

The developing brain is also sensitive to hormones in utero. Knowing this,
Norman Geschwind and his graduate student Albert Galaburda, now at Harvard
Medical School, made a controversial claim in 1985. They suggested that prenatal
testosterone slows the growth of certain areas of the left hemisphere and
facilitates the growth of corresponding regions of the right hemisphere. At the
time they wondered whether testosterone was partly to blame for such things as
left-handedness, dyslexia and autism (Archives of Neurology, vol 43, p
428).

Galaburda and his colleagues have since developed a way to induce selective
brain damage to the frontal lobe of newborn rats to mimic some of the symptoms
of dyslexia. Curiously, while male rats with this kind of damage have trouble
responding to rapidly changing sounds—much like dyslexic
humans—females don’t. “We induce the malformations in males and females,”
he says, “but only the males have trouble.”

It is clear that there is a genetic component to dyslexia. But Galaburda
thinks fetal testosterone plays a role too, by reducing plasticity in the young
brain, making males susceptible to brain malformations that females manage to
overcome. Intriguingly, when female rats are given extra testosterone, they too
show signs of dyslexia.

Manning hasn’t yet checked the finger lengths of human dyslexics to see
whether they also point to a testosterone link. But he has already checked out
the handedness idea, using a dexterity test. People are not always
straightforwardly right or left-handed: many have been trained to use their
right hand for writing, even if they are more skilled with the left. So Manning
and his colleagues tested how quickly 285 children could move 10 pegs from one
row of holes to another row five inches away, using one or the other hand.

Children with low 2D:4D ratios—suggestive of high exposure to
testosterone in the womb—are more likely to be quicker with their left
hands than the kids with higher ratios. This, he says, suggests that our degree
of left-handedness—and more generally the way the brain divides up tasks
between left and right hemispheres—may be influenced by hormone levels in
the womb.

Manning has begun examining autism too. He teamed up with Simon Baron-Cohen
and Svetlana Lutchmaya from the University of Cambridge, who have used samples
of amniotic fluid to directly measure the levels of hormones that babies are
exposed to in the womb. When the children reached their first birthday, the
researchers measured their vocabularies and ability to make eye contact. Poor
language skills and an unwillingness to make eye contact are early hallmarks of
autism. They found that babies who’d been exposed to high levels of testosterone
in the womb fared the worst.

“What we’re hoping to look at is whether finger ratios can be used as a proxy
for hormones,” says Lutchmaya. Amniocentesis is a risky procedure that only a
few mothers choose to undergo, she says. But by measuring finger lengths
instead, researchers can assess a random sample of children for possible early
signs of impaired language and social skill development. Currently, they are
checking the fingers of children for whom they have amniotic samples.

Meanwhile, Manning and Baron-Cohen have looked at the finger ratios of 49
children with firm diagnoses of autism, 23 with a mild form of the disorder
called Asperger’s syndrome, and their families. The researchers found that
autistic children tended to have very low 2D:4D ratios. Interestingly, children
with Asperger’s syndrome had ratios that fell between those of autistics and
unaffected children. “It fits exceptionally well with the theory,” says
Manning.

Thinking differently

Clearly genes play a role too in these conditions. But could fetal hormone
levels explain other cognitive differences between the sexes? Janel Tortorice at
Rutgers University in New Brunswick, New Jersey, thinks they may. She has
measured finger ratios in 40 gay women and found that their hands were
significantly different from those of heterosexual women—in fact, they
tend to resemble those of heterosexual men.

But she has also found differences in the way these women’s brains work.
“They have more masculine fingers and more masculine cognition,” she says. On
tests of spatial and verbal ability, lesbian volunteers perform more like men
than heterosexual women, she says. If this can be confirmed by further studies,
perhaps Manning’s most recent suggestion is not as outrageous as it sounds. He
claims that musical talent, too, is nurtured in the womb.

Manning recruited 54 male musicians from a British symphony orchestra. He
discovered that these men had significantly lower 2D:4D ratios than
controls—they had a very “masculine” ratio. Even more striking, when he
compared the top-ranked “first” musicians with their lower-ranked
colleagues—a measure of their relative ability—the former had
significantly lower 2D:4D ratios. Could testosterone really predispose the brain
to be more tuned in to music? Manning thinks so.

Musicians with short ring fingers and lesbians with long index fingers
needn’t lose heart, however. Even if fingers win a place in the pantheon of
diagnostic medicine, it’s unlikely that prospective employers or partners will
ever be able to predict our fortunes from our hands. Tortorice reminds us that
males tend to be taller than females. “But,” she says, “we don’t use height to
determine whether you’re a man or a woman.”

Fingers and toes have made a splash in the popular press at least three times
in the past three years—as an indicator of sperm counts, penis size and
homosexuality.

Can’t swim

August, 1998. At a meeting on human ethology in Vancouver, Canada, John
Manning from the University of Liverpool reports that men whose left hands
weren’t exact mirror images of their right hands are more likely to have low
sperm counts—and the sperm they do have often can’t swim. He also finds
that men with very low (“masculine”) finger length ratios tend to have higher
testosterone levels and more impressive sperm counts (Human
Reproduction, vol 13, p 3000).

For women, they conjecture, the opposite will be true: those with longer
index fingers will be more fertile, have higher oestrogen levels, and have
children at an earlier age.

Men with big feet

November, 1997. Nature publishes a short paper entitled “Of fingers,
toes and penises” which, not surprisingly, receives some media attention (
Nature, vol 390, p 29). In the letter, Takashi Kondo and his colleagues
from the University of Geneva, Switzerland, and the University of Michigan
Medical School in Ann Arbor report that a gene called Hox, which drives
the development of limb buds, is also responsible for the formation of the
penis. So that adage about men with big feet may be true.

Which way?

April, 2000. Marc Breedlove and his colleagues at the University of
California at Berkeley announce that “highly masculinised” hands are indicative
of more than just high fertility rates—homosexual men tended to show the
low ratio too (Nature, vol 404, p 455, and 91av, 1
April, p 5) though the finding wasn’t statistically significant.

A more striking, and statistically significant, finding was for gay women.
Like heterosexual men, they tended to have longer ring fingers. This suggests
that at least some gay women were exposed to higher than average levels of fetal
androgens, says Breedlove.