All together now. This week, the following markets have fallen after prolonged rallies: equities, equity volatility (measured by the CBOE Vix index), oil and the dollar.

This is not a coincidence. These markets are tightly linked. Optimism in each helps the others, and the correlations between them are disquietingly strong, just as they were before the crisis last year. Drawing arrows of causation between them, though, is tricky.