Final answer:
In 1931, during the Great Depression, the U.S. experienced deflation rather than inflation, which means both the GDP deflator and the inflation rate would indicate a decrease in price levels.
Step-by-step explanation:
The GDP deflator is a measure of price inflation within a nation's economy, reflecting the average prices of all goods and services. It is used to differentiate between nominal GDP, which is affected by inflation, and real GDP, which is adjusted for inflation. During the period of the Great Depression, particularly in the year 1931, the United States experienced deflation, not inflation. This can be inferred from historical data that show the price levels dropping significantly in the 1930s, which is a characteristic of a deflationary period. Hence, both the GDP deflator and the inflation rate for 1931 would have reflected a decrease in the average price level, as opposed to an increase that is usually implied by the term 'inflation.'