When I was in California last week, I met with a prospective client and did a  second-opinion financial review of his situation. He has $5mm in his company’s ESPP (employee stock purchase plan.) I can’t help but feel a bit dizzy, that feeling you get when you’re standing on the edge of a tall building without any protection.

I have a friend who was a senior engineer at MCI Worldcom. He also participated in this company’s ESPP.  In only a few years, Worldcom went from being a no-name, little known company to acquiring the second largest telecom at the time – MCI, and its stock price went up tenfold.  The value of my friend’s ESPP account went from $300k to over $3mm and he looked extremely smart by not diversifying at all.

The rest of the story you all know. MCI Worldcom filed for chapter 11 in 2002 due largely  to corporate fraud committed by their executives. Its stock price plummeted  to zero and my friend lost every dime in his ESPP.

So what exactly is an ESPP?

An ESPP enables a company employee to purchase company stock through payroll deduction.These  kind of plans are very popular among high-tech companies because they are considered a very effective way to align the interests of the employees and the firm.

A typical ESPP usually allows an employee to purchase company stock at a discount of up to 15% to 20%, and requires the employee  to  hold the stock for at least one year. After a year, the employee is allowed to diversify.

But few diversify, much to their financial peril. Here are the common reasons they don’t:

1. Inertia. Most people just have this deep-seated status quo bias. If things ain’t broke, why change? Payroll deduction is automatic, but to diversify the employee would actually need to create another brokerage account, move their  company shares there and sell them and buy other stocks or funds. This is just too much work for comfort.
2. Affinity. Most people also have a deep-seated affinity bias. They are familiar with the company, they like their colleagues, so they also like the company stock. It may be subconscious, , but they don’t want  to sell their stocks, since selling feels like separating.
3. Overconfidence. Since they work for the  the company, they think they know everything about the company.  They feel as though  they are in control, therefore a concentrated position doesn’t feel as risky to them.
4. Capital gain tax. Since shares are purchased at a discount, when they are sold, capital gains taxes have to be paid. Many people would rather not pay taxes.

All of these are not good reasons not to diversify. You may think that  since you work in the company, you are on top of things. Well, my friend who worked for MCI Worldcom thought so as well. So did those  employees who worked for Enron and Lehman Brothers and lost everything.

You don’t know what you don’t know. If you’ve read this far, you probably have a pretty sizable ESPP. Do me a favor, take this tiny step: put a date on your calendar to diversify your concentrated position. This little nudge might just be enough to help you overcome your inertia bias.