  Cultural traits such as words, names, decorative styles, and technical
standards often assume arbitrary values and are thought to evolve neutrally.
But neutral evolution cannot explain why some traits come and go in cycles of
popularity while others become entrenched. Here we study frequency-dependent
selection (FDS)--where a trait's tendency to be copied depends on its current
frequency regardless of the trait value itself. We develop a maximum-likelihood
method to infer the precise form of FDS from time series of trait abundance,
and we apply the method to data on baby names and pet dog breeds over the last
century. We find that the most common names tend to decline by 2%-6% per yr on
average; whereas rare names--1 in 10,000 births--tend to increase by 1%-3% per
yr. This specific form of negative FDS explains patterns of diversity and
replicates across the United States, France, Norway and the Netherlands,
despite cultural, linguistic and demographic variation. We infer a fixed
fitness offset between male and female names that implies different rates of
innovation. We also find a strong selective advantage for biblical names in
every frequency class, which explains their predominance among the most common
names. In purebred dog registrations we infer a form of negative FDS that is
consistent with a preference for novelty, in which each year's newest breeds
outgrow the previous by about 1%/yr, which also recapitulates boom-bust cycles
in dog fanciers. Finally, we define the concept of effective
frequency-dependent selection, which enables a meaningful interpretation of
inferred FDS even for complex mechanisms of evolution. Our analysis generalizes
neutral evolution to incorporate pressures of conformity and anti-conformity as
fundamental forces in social evolution, and our inference procedure provides a
quantitative account of how these forces operate within and across cultures.
