Earlier this year Albert-László Barabási of the University of Notre Dame and Harvard University showed that the length of time it takes people to reply to emails can be described by a power law. Now Barabási and Joāo Gama Oliveira, who is based at the Universidade de Aveiro in Portugal and Notre Dame, have shown that the response times of Einstein and Darwin to letters can also be described by a power law, albeit with a different exponent. The results suggest that there is a global pattern for human behaviour in correspondence that also applies to famous scientists like Einstein and Darwin (Nature 437 1251).

Both Darwin (1809-82) and Einstein (1879-1955) wrote and received many letters during their lives: Darwin sent at least 7591 letters and received 6530, while Einstein sent more than 14500 and received 16200. Oliveira and Barabási have shown that although times have changed with the advent of email, communication dynamics have not. However, the time scales involved are much shorter for email.

Oliveira and Barabási analysed data about letters in the Darwin Correspondence Project and the Einstein Papers Project and calculated the response time, τ - the number of days between Einstein or Darwin receiving a letter from someone and then sending a reply. Oliveira and Barabási found that the probability that a letter will be replied to in τ days, P(τ), can be described by a power law, P(τ)≈τ, where α=1.45±0.1 for Darwin and 1.47±0.1 for Einstein, compared with α=1 for email. This means that Darwin and Einstein sent more than half of their replies within ten days, but sometimes took months, or even years, to reply.

"What I find most remarkable is that the response pattern observed for both Darwin and Einstein is similar, despite the fact that they both lived in different eras," Oliveira told PhysicsWeb. "This suggests that the observed pattern is not characteristic of either of them, but is rather a global pattern for human behaviour." Oliveira and Barabási have since observed a similar pattern in the letters of Sigmund Freud.