Final answer:
The original Phillips curve failed to consider people's inflation expectations, leading to inaccuracies in predicting economic conditions such as stagflation, where both inflation and unemployment rise together.
Step-by-step explanation:
The weakness of the original Phillips curve is that it ignored expectations.
The original Phillips curve posited a stable, inverse relationship between inflation and unemployment, suggesting that policymakers could choose a point along the curve to achieve a desired balance between the two economic variables. However, this view neglected the impact of people's expectations about inflation. Over time, as people came to anticipate inflation, they would adjust their behavior accordingly, diminishing the tradeoff. This adjustment leads to a shift in the aggregate supply curve and the Phillips curve, rendering the initial tradeoff unstable.
The omission of expectations meant that the Phillips curve could not accurately predict or explain the phenomenon of stagflation, a situation where high inflation and high unemployment occur simultaneously. The original curve assumed that these two factors moved in opposite directions, but the 1970s oil crisis and subsequent economic conditions demonstrated that both could rise at the same time, due to supply shocks and changes in inflationary expectations.