Why lumpsum wins on average
Markets rise more often than they fall, so money invested earlier is exposed to more up-days. Vanguard's well-known study found that investing a lumpsum immediately beat dollar-cost averaging the same sum over 12 months in roughly two-thirds of historical periods, by an average of a few percent. The reason is simple: with a SIP, part of your money sits in cash for months earning little, missing the compounding that the lumpsum captures from day one.
Why a SIP is still the right default for most people
The lumpsum edge assumes you have a lumpsum. Most people invest from a salary, so a SIP is not a strategy choice — it is the only option, and a good one. It enforces discipline, removes the temptation to time the market, and means you are never the investor who put their entire savings in the week before a 30% drawdown. The regret-minimising choice and the average-maximising choice are not always the same.
The honest tie-breaker: inflation and the real number
Whichever you pick, the corpus you see is a nominal number. Over a 20–30 year horizon, inflation can halve its purchasing power. The comparison that actually matters is not 'SIP vs lumpsum corpus' but 'real corpus vs your real goal'. Size the target in today's money first, then choose the contribution method that gets you there given your cashflow.