Decrease in popular culture turnover rate

[This post is the first in a series I’d call “Things that I probably will not develop in a proper paper, but I find interesting enough to write here”]

One way to quantify change in cultural dynamics is to measure the turnover rate of a particular domain. The turnover (z) is the number of new items that enter, after a certain amount of time t, in an ordered list of size N. What does this mean? A straightforward example of turnover is the new entries in a Top-list Chart. This week, for example, 4 new singles entered in the BBC UK Top-40 Single Chart (see here – of course the actual number of new entries will change from week to week).  So, for the week starting 8 March 2015, z=4 (the number of new items that enter…), t=1 week (…after a certain amount of time…), N=40 (…in an ordered list of size N). Notice that, with this information, one can calculate the turnover for all N from 1 to 40 (for example, this week, the 4 new entries are at the 1st, 7th, 13th, and 18th place, so, for, say N=10, z would be equal to 2).

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Detecting cultural transmission biases in real-life dynamics

Many studies of cultural evolution have focused on how transmission biases affect the likelihood of cultural traits of being transmitted. The concepts are quite intuitive. An useful distinction is between content biases, when the intrinsic features of a cultural trait make it more likely to spread (the effectiveness of a tool may be a content bias, but also a sexual hint in an image), and context biases, when the likelihood is determined by the context, as when we tend to dress as our friends/coworkers (conformist bias; but one can do the opposite, and prefer unpopular cultural traits), or as when I was trying to have a young Axl Rose haircut (prestige bias – see also my picture on the left).

Some interesting works in cultural evolution have examined, with analytical and simulation models, the adaptivity of transmission biases (e.g. did my Axl Rose haircut make me rich and/or attractive? It did not, but, on average, prestige bias may be useful) or examined the transmission biases long-trend dynamics in idealised situations (e.g. how fast will a new cultural trait “invade” a population of conformist individuals? or of anti-conformists? etc.). Other works investigated, in controlled experiments, if people are indeed subject to transmission biases when copying from others (they are, with caveat).

What is partly missing is an understanding of the impact of transmission biases in real life cultural dynamics. We recently had a paper accepted in Evolution and Human Behavior that tackles this problem.  In brief (much more is in the paper!): (1) we focused on the turnover of popular traits, i.e. on how many new traits enter in a top-list of a certain size for a certain cultural domain (like here); (2) we used some predictions on how the turnover would look like  if there were no biases, that is, if everybody would just copy others at random (neutral model of cultural evolution); and (3) we showed how these predictions differ if biases are instead present.

turnover

The turnover of some cultural domains, for example recent baby names in USA, looks like the red line in the figure above, signalling that people tend to prefer relatively un-common names. The turnover of others, like early baby names, musical preferences of Last.fm users who subscribed to genre-based groups (“80s Gothic”, “Acid Jazz”) , or usage of colour terms in English language books, looks instead like the blue line, signalling a conformist bias, or a content-based bias (which I call “attraction”).

Overall, turnover can be calculated when we have periodical top lists, or, more generally, when we can “count” the frequency of items trough time. Given the ubiquity of this kind of information in digital form, one can use this methodology to infer individual behaviour from population-level, aggregate, data for several cultural domains.

Acerbi, A. and Bentley, R.A. (2014), Biases in cultural transmission shape the turnover of popular traits, Evolution and Human Behavior, in press.