  It is a long-standing question whether human sexual and reproductive cycles
are affected predominantly by biology or culture. The literature is mixed with
respect to whether biological or cultural factors best explain the reproduction
cycle phenomenon, with biological explanations dominating the argument. The
biological hypothesis proposes that human reproductive cycles are an adaptation
to the seasonal cycles caused by hemisphere positioning, while the cultural
hypothesis proposes that conception dates vary mostly due to cultural factors,
such as vacation schedule or religious holidays. However, for many countries,
common records used to investigate these hypotheses are incomplete or
unavailable, biasing existing analysis towards primarily Christian countries in
the Northern Hemisphere. Here we show that interest in sex peaks sharply online
during major cultural and religious celebrations, regardless of hemisphere
location. This online interest, when shifted by nine months, corresponds to
documented human birth cycles, even after adjusting for numerous factors such
as language, season, and amount of free time due to holidays. We further show
that mood, measured independently on Twitter, contains distinct collective
emotions associated with those cultural celebrations, and these collective
moods correlate with sex search volume outside of these holidays as well. Our
results provide converging evidence that the cyclic sexual and reproductive
behavior of human populations is mostly driven by culture and that this
interest in sex is associated with specific emotions, characteristic of, but
not limited to, major cultural and religious celebrations.
