What is the Dollar-Cost Effect?
The dollar-cost effect is the mathematical outcome of investing a fixed dollar amount at regular intervals. Because you buy more shares when prices are low and fewer when prices are high, your average cost per share is always lower than the simple average of the prices at which you purchased. This is a consequence of the harmonic mean being less than the arithmetic mean when values vary.
Numerical Example
Suppose you invest $300 monthly over three months when a fund is priced at $10, $6, and $10. You buy 30, 50, and 30 shares respectively, totaling 110 shares for $900. Your average cost is $8.18 per share, while the average price was $8.67. The dollar-cost effect saved you $0.49 per share, or about 5.6%. The effect is more pronounced when prices are more volatile, as greater price swings create larger differences between the harmonic and arithmetic means.
Key Considerations
The dollar-cost effect does not guarantee profits. If prices decline steadily without recovering, you accumulate shares at progressively lower prices but still face losses. The effect works best in volatile markets that ultimately trend upward. It is a natural byproduct of disciplined periodic investing rather than a strategy to be actively pursued.