Is it dangerous to invest a big lump sum all at once right before a downturn?

It feels dangerous, but the data leans the other way. Vanguard's research found that investing a lump sum immediately beat dollar-cost averaging it in over roughly two-thirds of historical periods, because markets rise more often than they fall, so money on the sidelines usually misses gains. Waiting for a 'better' entry point is a form of market timing that often backfires.

That said, math isn't everything. If deploying it all at once and then watching a 20% drop would make you panic-sell, spreading it over a few months buys emotional insurance that's worth the small expected cost. A reasonable compromise: invest a large chunk now to capture likely gains, and average in the rest over 3-6 months. The key is that the money ends up invested according to your target allocation, not sitting in cash indefinitely out of fear.