Business cycles refer to fluctuations in overall economic activity, moving between periods of expansion and contraction. A contraction or recession occurs when aggregate economic activity declines until reaching a trough, then economic activity increases during an expansion until reaching a peak. In the US, recessions were common before World War 1, with the longest lasting 65 months from 1873-1879. The worst contraction was the Great Depression of the 1930s, when real GDP fell 30% and unemployment rose to 25%. After World War 2, economists believed business cycles became less severe, with average contractions shorter and expansions longer than before 1929.