Novelty, popularity, and emergent neutrality: bias in the choice of baby names and lessons for analyzing cultural data

Neutral evolution assumes that there are no selective forces distinguishing different variants in a population. Despite this striking assumption, many recent studies have sought to assess whether neutrality can provide a good description of different episodes of cultural change. One approach has been to test whether neutral predictions are consistent with observed progeny distributions, recording the number of variants that have produced a given number of new instances within a specified time interval: the classic example is the distribution of baby names registered within the course of a year. Here we develop new methods to analyze the shape of progeny distributions. Previous work had approximated the neutral progeny distribution as a power law with an exponent varying with population size and innovation rate. Using an overlapping generations model we show that the progeny distribution consists of two phases: a power law phase with an universallyapplicable exponent of -3/2, followed by an exponential cut-off for variants with very large numbers of progeny. Maximum likelihood estimations of the model parameters then provide a direct way to parameterize the neutral model. We apply our approach to a data set of baby names from Australia and find that while neutrality provides a plausible description of the progeny distribution of abundant variants, rare variants deviate from neutrality. This indicates that analyses based on only the most popular variants, as is often the case in studies of cultural evolution, can provide misleading evidence for underlying transmission hypotheses. To determine whether the progeny distribution is a meaningful statistic to distinguish between alternative processes of cultural transmission we analyze the effects of novelty biases. We show that a kind of anti-novelty bias, where new variants are at a disadvantage by virtue of their novelty, is able to replicate more closely the complete progeny distribution of the Australian data set. Article submitted to Royal Society TEX Paper ar X iv :1 70 2. 08 50 6v 1 [ qbi o. PE ] 2 7 Fe b 20 17 2 JP O’Dwyer, A Kandler