The expectation of a seasonal pattern in a variable can cause an actual seasonal effect in that or some other variable, since expectations can lead to plans that then ensure seasonality. An example is toy production in expectation of a sales peak during the Christmas period. Without the expectation-planning aspect, the seasonal pattern may still occur but might be of a different shape or nature. Expectations may arise because it has been noted that the series being considered has in the past contained a seasonal pattern, or because it is observed that acknowledged causal series have a seasonal component.
Handbook on quarterly national accounts, 1999 Edition, Eurostat, p.201