I asked myself this question this month as well. I'm investing the $5500 over 3 weeks. In thirty years the small variations in stock prices this month (and year) won't matter much.

I put the $5,500 max in in January each year. We also put in $6,500 for my husband each January.

He is looking at the risk of investing all when the market is high, but what of the risk of failing to invest all when the market is low?

Invest it all now. The average monthly and yearly stock market return is positive, so in any given month/year the early investment strategy is more likely to beat the averaging strategy.

Unless you have a macro thesis (your prediction of where the market is going) in which case take action commensurate with your conviction.

But don't over think it - the bottom line is, invest it all and invest it well!

Dollar Cost Average... some months will do well, some not. Average it out by dividing it up, usually monthly.

Last year, I put $5k in an index fund in Jan (into my IRA)
My husband chose to put in smaller amounts quarterly, saying that way he could minimize the risk of allocating all the money at a time the market might be high.

But last year, the market went up generally, so my return was better than his.
But I confess, it was just dumb luck on my part.

If we have the $5k each in Jan to put in for the year, do you think it's better to do so?