|
PAIN, EVOLUTION
AND THE PLACEBO RESPONSE
First published in Behavioral and Brain Sciences 25 (4);
459-460 (2002).
For a PDF of this article, click here.
INVITED COMMENTARY on 'Facial expression of pain: an evolutionary
account', by Amanda C. de C. Williams
Dylan Evans
Department of Mechanical Engineering
University of Bath
Bath BA2 7AY
United Kingdom
Abstract:
Williams argues that humans have evolved special purpose adaptations
for eliciting medical attention from others, such as a specific facial
expression of pain. She also recognises that such adaptations would
almost certainly have coevolved with adaptations for providing and responding
to medical care. The placebo response may be one such adaptation,
and any evolutionary account of pain must also address this important phenomenon.
Main text:
Williams argues that among the evolved human facial expressions there
is a distinct facial expression of pain. The function of this state,
she claims, is to elicit social assistance of a medical kind. The
plausibility of this claim depends on how long medical care has been around.
Unfortunately, we are extremely ignorant about the exact age of medicine.
It must have originated after the human lineage had already diverged from
that of the chimpanzees, since chimpanzees do not practise medicine, if
by medicine we mean the provision of special care to a sick individual by
others. Primatologists have observed many cases in which a chimpanzee
takes care of himself when ill or injured, sometimes in quite elaborate ways,
such as consuming plants with medicinal properties or dabbing leaves on bloody
wounds, but they have never seen one chimp providing this sort of medical
assistance to another. Chimpanzees do spend long hours picking the
ticks off each other’s backs, which could, perhaps, be regarded as a kind
of preventative medicine, but therapeutic medicine seems to lie outside
their behavioural repertoire.
Archaeological evidence of ancient medical practices does not appear
until relatively late. Ancient texts from Mesopotamia and Egypt provide
written evidence that sophisticated medical practices were well established
by 1700 BC (Porter 1997), but evidence of the existence of medicine prior
to the advent of writing is much harder to come by. One rare example
is the existence of skulls with small holes surrounded by calluses that indicate
that trephining was being performed in places as far apart as France and
the Pacific by 5,000 BC. This is an operation involving cutting a small
hole in the skull and scraping away portions of the cranium. If such
a complex operation was being performed 7,000 years ago, it is a fair bet
that more primitive forms of medicine were being practised earlier, but
how much earlier is hard to say.
We know, then, that medicine – the provision of special care to the sick
by others – must have originated some time between five million years ago
and 10,000 years ago. That, of course, is a very large time window.
It is so large, in fact, as to leave us ignorant on the vital question
of whether or not there has been enough time for natural selection to shape
specific adaptations for medical care. If medicine originated towards
the beginning of this window, shortly after the hominid lineage branched
off from that of the chimpanzees, then there would certainly have been time
for the human brain to have developed special purpose mechanisms for eliciting,
providing and responding to medical help. If, however, medicine only
started towards the end of this time window, when our ancestors were already
fully human, then there would not have been time for any such special-purpose
‘medical adaptations’ to have evolved.
Still, even if we are ignorant on this point, we can still explore each
of the alternatives. The first possibility is that medicine is a few
million years old, and that humans have evolved special psychological and
physicological mechanisms that are for eliciting, providing and responding
to the provision of medical attention. Williams concentrates on adaptations
for eliciting medical care – in particular, on the facial expression of pain
– but she also points out that such adaptations would almost certainly have
coevolved with adaptations for providing and responding to medical care.
Williams draws on the work of the late Patrick Wall, particularly on
his claim that pain is a ‘need state’, like hunger and thirst (Wall 1999b).
Need states are terminated by specific consumatory acts; hunger by
eating, thirst by drinking, and so on. Pain, presumably, is no different.
Withdrawing one’s hand rapidly from a hot stove is a consumatory act that
terminates one sort of pain; keeping a sprained ankle still is a
consumatory act that terminates another. Crucial to Wall’s argument,
however, is that pain can sometimes be terminated simply by care and attention
from others. It is this addition of a purely social event to the list
of various consumatory acts relevant to pain that makes human pain such
an evolutionary novelty.
Wall’s claim about the relevance of social support to pain relief is
supported by studies that have investigated the anti-inflammatory effects
of fake ultrasound (Hashish, Harvey et al. 1986; Hashish, Hai et al. 1988).
One of these studies found that the placebo response was only triggered
when the fake ultrasound was applied by someone else. When exactly
the same physical stimulus was applied by the patients to their own faces,
the swelling was not reduced. This suggests that the mere provision
of social support can be sufficient to trigger the placebo response.
Perhaps this is the result of natural selection wiring up the pain generating
circuits in the brain to inputs from the neural regions that are sensitive
to social support. This, of course, presupposes that medical care has
been a feature of our environment for long enough to enable such evolutionary
change to take place.
If Wall’s theory is right, and natural selection has designed specific
brain circuits to feed information about the social environment into the
circuits that generate pain, such circuits must confer some evolutionary
advantage on those who have them. It is hard, however, to see what
this advantage might be. As Williams points out, pain is a vital
protective mechanism, and those who lack the capacity for pain do not survive
very long. What possible advantage could there be in having a mechanism
that shuts down pain when medical care is provided? Surely it would
be better to make the sensation of pain autonomous, independent of such social
factors?
Not necessarily. There are costs as well as benefits associated
with pain (Humphrey 2000). In particular, high levels of pain can actually
slow down the healing process. When one is alone, the protective value
of pain might outweigh the disadvantage of slowing down the healing process,
but when others are taking care of you, the cost-benefit ratio may change.
In particular, when others can protect you, pain might not be so vital.
In effect, medical care might allow the patient to offload the protective
function of pain onto others: self-defence is unnecessary when other
people are around to do the defending for you. If this is true, then
a person whose brain was capable of shutting down pain when it detected
the presence of medical help might actually have an advantage over someone
whose brain lacked this capacity (Evans 2003).
References:
Evans, D. (2003). Placebo: The Belief Effect. London, Harper Collins.
Hashish, I., H. K. Hai, et al. (1988). "Reduction of postoperative pain
and swelling by ultrasound treatment: a placebo effect." Pain 33: 303-311.
Hashish, I., W. Harvey, et al. (1986). "Anti-inflammatory effects of
ultrasound therapy: evidence for a major placebo effect." British Journal
of Rheumatology 25: 77-81.
Humphrey, N. (2000). Great expectations: the evolutionary psychology
of faith-healing and the placebo effect. The Mind Made Flesh: Essays from
the Frontier of Evolution and Psychology. N. Humphrey. Oxford, Oxford University
Press, 2002, chapter 19.
Porter, R. (1997). The Greatest Benefit to Mankind: A Medical History
of Humanity from Antiquity to the Present. London, Harper Collins.
Wall, P. D. (1999b). Pain: The Science of Suffering. London, Weidenfeld
& Nicholson.
This page was last updated: 16 September 2003.
|
|