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Are stock index fund likely to keep being a reliable long-term investment option?
The idea behind investing in index funds is that you will not under perform the market but also at the same time not over perform against the market either. It is meant for those (majority of the investing population) who do not or cannot invest more time in actively researching different investment options. So even considering for a moment that the yields on the index funds will drop significantly in the future, since the fund is supposed to be replication of the whole market itself, the market too can be assumed to be giving significantly lower future yields. In my opinion the question that you ask is confusing/contradictory because, its like pegging the fund performance to an avg and then asking if it will be higher or lower in the future. But rather its always going to be exactly the average, even if the absolute yields turn higher or lower