Introduction to Business Cycles

Business Cycle Definitions and Phases

Arthur Burns and Wesley Mitchell (1946) defined business cycles as fluctuations in aggregate economic activity where multiple economic sectors expand and contract together in a recurring, non-periodic pattern. A standard business cycle progresses through four distinct phases: trough, expansion (boom), peak, and contraction (recession). An alternative analytical perspective decomposes total output into a long-run potential output trend ($Y^P$) and short-run deviations known as the output gap ($Y - Y^P$).

Empirical Characteristics and Volatility

Over long horizons, real GDP growth averages 3–3.5 percent per year, though it is subject to large fluctuations in the short run. Aggregate consumption and investment co-move alongside GDP, but with differing degrees of volatility. Consumption growth is less volatile than overall GDP growth, whereas investment growth displays substantially higher volatility across business cycles.

Unemployment and Okun's Law

Labor market dynamics fluctuate closely with aggregate output: the unemployment rate rises during recessions and falls during expansions. Okun's law captures this relationship by identifying a strong negative correlation between real GDP growth and changes in the unemployment rate, measured at a correlation of $-0.81$.

Co-Movement, Timing, and Indicators

Macroeconomic variables demonstrate systematic co-movement and timing relative to cycle turning points. Variables are classified as procyclical, countercyclical, or acyclical based on whether they move in the same, opposite, or uncorrelated direction relative to aggregate activity. Based on timing, variables are leading, lagging, or coincident depending on whether their turning points occur before, after, or at the same time as the business cycle. The Conference Board combines 10 leading variables into an index to forecast shifts in economic activity 6–9 months into the future.

Key Business Cycle Definitions

• Business Cycle: Fluctuations in aggregate economic activity in which multiple economic sectors expand and contract together in a recurring, non-periodic pattern.

• Potential Output ($Y^P$): The long-run trend level of total real output ($Y$) when economic resources are fully utilized.

• Output Gap ($Y - Y^P$): The short-run deviation of actual economic output ($Y$) from potential output ($Y^P$).

Business Cycle Phases

• Peak: The maximum point of aggregate economic activity marking the transition from expansion to contraction.

• Contraction (Recession): A phase characterized by widespread decline in economic activity between a peak and a trough.

• Trough: The lowest point of aggregate economic activity marking the transition from contraction to expansion.

• Expansion (Boom): A phase characterized by widespread growth in economic activity between a trough and a peak.

Co-Movement & Timing Classifications

• Procyclical Variable: An economic variable that moves in the same direction as aggregate activity (rises during expansions, falls during contractions).

• Countercyclical Variable: An economic variable that moves in the opposite direction of aggregate activity (falls during expansions, rises during contractions).

• Acyclical Variable: An economic variable whose movements do not systematically correlate with aggregate economic activity.

• Leading Variable: A variable whose turning points occur prior to the turning points of the overall business cycle.

• Coincident Variable: A variable whose turning points occur at the same time as the turning points of the overall business cycle.

• Lagging Variable: A variable whose turning points occur after the turning points of the overall business cycle.

Quantitative Relationships & Indicators

• Okun's Law: The empirical rule identifying a strong negative correlation (measured at $-0.81$) between real GDP growth and changes in the unemployment rate.

• Index of Leading Economic Indicators: A composite index published monthly by The Conference Board combining $10$ leading variables to forecast economic shifts $6$ to $9$ months into the future.