I’ve just inherited a six-figure sum and am interested in investing using the Couch Potato approach. What is your opinion on entering the market now, given it’s at a record high? — C.T.

This is the most common question I hear these days. It took almost four years for investors to finally admit we’re in a charging bull market—US markets are up more than 150% since March 2009—but most of the chatter now seems to be about a looming correction. This week, one noisemaker even came up with 21 stock market warning signs giving global investors cold sweats. Hard to believe he could only come up with 21.

Even if the media weren’t shouting like this, investing a six-figure lump sum would still be difficult. That’s especially true with an inheritance, since it often takes a while for beneficiaries to feel like that money really belongs to them. But in general—assuming you’ve done a careful risk assessment and have a target asset allocation in mind—it’s usually best to invest the money immediately. Here’s why:

Not everything is at an all-time high. While US markets have been on fire recently, Canadian equities are nowhere near the peak they reached in 2008. International equity ETFs (both developed and emerging markets) are priced lower than they were in 2007.  And when equities do suffer a correction, bonds will likely offset some of those losses, becoming expensive as they do so. That’s the nature of a diversified portfolio: it’s unlikely your timing will ever be good or bad across all asset classes.

A pullback is coming—but we have no idea when. The danger of waiting for a pullback in the markets is simple: it can take a very long time. The Wall Street Journal reported recently that “there have been 35 periods where the S&P 500 went longer than 165 days without a 5% decline.” We’re in one of those six-month stretches now, and US markets are up more than 20% over that period. If you’ve been sitting on the sidelines for months—and people have been worried since at least the beginning of the year—the opportunity cost was enormous.

It’s not just that it’s impossible to time the stock market: timing other markets is mug’s game, too. Let’s remember the widely predicted interest rate hikes that were supposed to kill bonds (the DEX Universe Bond Index is up 5.6% over the last 12 months, and 6.5% annually over the last three years) and the housing crash that’s been looming forever, though prices continue to rise. Forecasts are usually wrong, period.

When the correction comes, you won’t want to buy. Investors often talk about how a sharp decline in prices is a “buying opportunity.” But they should ask themselves whether they really have a track record of buying after pull-backs.

When the S&P/TSX 60 plummeted almost 20% between March and September 2011, did you enthusiastically load up on more? During the prolonged debt crisis in Europe, were you topping up your EAFE fund? Or were you “waiting for things to settle down”? It’s easy to talk about buying low, but it’s much harder to actually do it.

Once you’re fully invested, then what? Let’s look at this question from a different perspective. Instead of having received a big inheritance recently, let’s say it arrived several years ago and you used it to build a Couch Potato portfolio with a mix 60% stocks and 40% bonds. You’ve been rebalancing regularly, so your portfolio is right on target today. Would you sell everything and go to cash now because markets are at an all-time high? If the answer is no, why would you prefer to keep your inheritance in cash today rather than investing it?

There’s no question that your entry point will have a significant effect on your portfolio’s long-term performance—especially if you don’t plan to continue adding money. Unfortunately, there simply is no way of determining the “right time” to enter the markets except in hindsight.