I hear people say this and I can see why they think their logic is sound. Certainly returns after hitting all time highs have to be lower than when we can 'buy at a discount', right? So I thought I'd share some research on the subject...
In the last 90 years, or 1,080 months, 75% of annual returns are positive for the S&P 500. Meaning, if we pick any month historically, there's a 75% chance that we earned a positive rate of return one year after and a 25% chance that the market was lower one year after. But what about in the months where we set new all time highs? We have reached new all time highs in 319 of the 1,080 periods, which is just under 30% of the time. Of those months, 80% delivered positive one year returns and 20% of those months delivered negative one year returns.
So, this idea that new market highs are a reason to not invest is silly. Historically the odds of success have actually been higher in months where we reach all time highs.