New Research Demonstrates That Social Interaction Determines Left Or Right-side Bias

Why aren’t left and right-handers equally common? New research* demonstrates that the prevalence of bias or handedness in one direction (Lateralisation) is likely to result from social selection pressures, rather than mere evolutionary chance or genetics. The research is published in Proceedings B, a learned journal published by The Royal Society.

We have long known that the two sides of the brain perform different functions – the left hemisphere for language and the right for visual-spatial functions. Scientists have also agreed that the brain works more efficiently when it is asymmetric. One consequence of brain asymmetry is lateralisation – the preferential use of a limb or eye – which is found not just in humans but the majority of vertebrates, including fish, sheep and primates. What has so far been unexplained is that the proportion in a population of these lateralisations is often not the random 50:50 one would expect but ranges from 90:10 to 65:35 depending on the species.

Taking the example of predator/prey interactions, scientists have used the mathematical ‘theory of games’, to show that ‘deciding’ to have the same direction of asymmetry as other individuals in the group could be an advantage. Individually, prey that can escape more effectively in one direction would have an advantage, but that the direction of that escape would not matter so the proportion would be 50:50. However in a group situation, the majority of prey get protection by keeping together and all escaping in the same direction, but pay a cost because predators can more easily predict this. A minority of prey manages to enjoy the same escape probability by trading off protection from the group with the advantage in the face of predators. This leads to an established and predictable proportion of lateralisation in the population.

The scientists believe that by applying this technique to other types of social interaction, they can explain many other unequal proportions found in populations, perhaps even right and left-handedness in humans as situations where there is both need for coordination between individuals and antagonistic interactions between those individuals due to lateralisation.

Dr Giorgio Vallortigara, one of the scientists who carried out the research says: “The traditional view for the left and right bias in animals is that it helps avoid duplication of neural circuitry with the same functions and decreases the interference between different functions. Our research offers a solution to the riddle of the emergence and maintenance of unequal proportions of individuals with opposite lateralization (left and right bias) and a bridge between neuropsychology and evolutionary biology. Social factors are likely to have been crucial in shaping the population structure of these biases,” he says.

*The evolution of brain lateralization: a game-theoretical analysis of population structure, by Dr Giorgio Vallortiga, University of Trieste and Dr Stefano Ghirlanda, Stockholm University.

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