Forgive me if you've heard this warning before, but it's simply too important to not repeat it: If you own a significant amount of your employer's stock, you are taking a dangerous — and unnecessary — risk with your financial future.

That's the takeaway of Your 401(k) Is Not Safe At Home, a recent Wall Street Journal column by Jason Zweig. Zweig covers a lot of ground that is likely familiar to SMI readers, given that SMI has covered this issue of employees loading up on their employer's company stock numerous times over the past couple decades (see Is Holding Company Stock in Your 401(k) Risky or Advantageous?). But he includes some new examples and statistics that continue to blow my mind.

First of all, it's clear that a huge number of American workers have absolutely no idea what they're doing when it comes to investing. That's not a put down from an elitist investment writer — it's an acknowledgment of a very scary fact, given that today's worker is more or less handed the keys to their own financial future (via a company 401(k) plan or similar) and told to figure it out on their own. Consider this statistic Zweig reports:

Holding hundreds of unfamiliar companies feels a lot scarier than investing in the one you show up to work at every day. A recent survey of 1,500 people found that 51% believed that a diversified bundle of 10 investments is more volatile than a portfolio consisting of only a single stock.

That's absolutely false. But starting from that faulty premise, it's not surprising that so many workers still have so much of their retirement savings concentrated in their employer's stock.

Zweig focuses specifically on the retirement plan of Kinder Morgan, the Houston-based energy pipeline company. After reviewing some of the company's recent stock price fluctuations, he sums up the problem of being too concentrated in employer stock succinctly:

Every investor’s portfolio consists of financial and human capital. Your financial capital is cash, bonds, stocks and the like; your human capital is the income you will earn from your work over your lifetime. The more your financial and human capital overlap, the more vulnerable you are to risks outside your control.

In 2015, home prices in Houston fell by more than 7% in the second half of the year. A worker with most of his retirement money in Kinder Morgan stock could end up with a shriveled nest egg and a house worth less than he paid for it, all at the same time.

And while he doesn't explicitly state it, the worst-case scenario is actually one significant step worse than that: a shriveled nest egg, underwater mortgage, and out of a job, all at once due to negative events hitting your company.

As we've reported in recent years, things are improving on this issue, albeit slowly. Zweig reports that "under 7% of [companies offering retirement plans] have at least half their assets in company stock, down from 17% a decade ago." But that still means that almost 7% of these plans have at least half their assets in company stock!

Don't get caught by this most common of traps. If you have the opportunity to buy stock in your employer at a discount, make sure you regularly prune your holdings to gain needed diversification. If your plan automatically matches your contributions in company stock, or otherwise makes their stock available in your retirement plan, learn the rules regarding how soon you can sell it and shift the money into other holdings.

The responsibility for your future financial condition lies squarely on your shoulders. The old model of the company taking care of investing for their employees and paying them retirement pensions is largely a thing of the past. So make sure you understand at least the basics of what's required of you. Avoiding holding too much of your employer's stock is one example, but there is obviously much more to successful investing than that. If you're not already an SMI member, check out our free workship, The Four Essential Keys to Becoming a Confident, Successful Investor.