Tit-for-tat explained

11 July 2003

Researchers at the Institute of Neurology, University College London, have discovered the neurological basis of force escalation in tit-for-tat situations. The team, led by Wellcome Senior Research Fellow Sukhwinder Shergill, used a motor to apply a fixed force to the finger of the first participant, who was then told to apply the same force to the other person's finger. Over eight turns, the force applied increased a massive 14-fold. Participants always responded with a force at least one-third greater than the force applied to them.

Although it is well known that a system in the brain de-emphasises the effects of our own actions, this is the first time it has been measured experimentally. It is likely that just before someone makes a movement, signals are sent to a specific brain region to warn it what to expect. This may enable the brain to distinguish between external and self-provoked forces (a mechanism that may also explain why it is impossible to tickle yourself).

The lack of accuracy in assessing directly applied forces may have implications for contact sports such as boxing, where impacts may be greater than participants think. Similarly, the researchers suggest, parents may be inadvertently smacking children harder than they intend.

The findings may also be relevant to people affected by schizophrenia, who can have difficulty in distinguishing between external and internally generated stimuli. The effect could underlie the experience of hearing voices - inner speech that, without the signals identifying it as self-generated, is experienced as coming from someone else. Dr Shergill has recently used brain imaging to show that a related phenomenon occurs in people with schizophrenia experiencing auditory hallucinations.

The tit-for-tat results were published in Science, 11 July 2003 (Vol. 301: 187).

External links

• Two Eyes for an Eye: The Neuroscience of Force Escalation, Science 301, 187 (11 July 2003)
• Institute of Neurology at University College London: Affiliation of Dr Sukhwinder Shergill