Research shows that investing a lump sum into the S&P 500 all at once beats dollar-cost averaging about 70% of the time, even when buying near record highs. However, spreading purchases over 3 months may help risk-averse investors psychologically weather significant market downturns, though it typically results in lower returns.
Axe note: Putting most of your cash into an S&P 500 index fund upfront beats spreading purchases out, unless you struggle with market swings.