The words you use when you communicate with customers, colleagues, and bosses signal whether you're a power-player or a pushover.

If you agree to do something, for example, "I'll try to do that" subtly signals that you plan to fail. By contrast, "I will definitely do that" signals that you'll get the job done no matter what.

Or, to quote a popular, though fictional, business guru: "Do. Or do not. There is no try."

The same thing is true when you need to say no, like to an unreasonable request from a customer, colleague or even your boss.

According to a recent article in the Journal of Consumer Research, using the phrase "I can't do that" implies that you're either incompetent (i.e. "I lack the ability to do that") or a pawn of forces outside of your control (i.e. "They won't let me do that.")

By contrast, using the phrase "I don't do that" implies that you're perfectly competent--in the sense that you possess the ability to fulfill the request--but that you've decided decision not to fulfill the request, on principle.

For example, suppose a customer asks for a discount after the deal has been negotiated. This is a tricky situation because customers use such requests to test if they've gotten the best deal. If you cave, they know you were overcharging, so you lose their trust and open the door to further requests.

There's a very different tone to the conversation if you fielding the request with "can't" rather than "don't." Compare these two versions:

Version 1:

  • Customer: I want a 30% discount or the deal is off.
  • You: I can't approve a discount that big.
  • Customer: Why not?
  • You: Well, I've already given you a discount.
  • Customer: But somebody has the authority, right? Who? Your boss?
  • You: Yeah, I guess, maybe.
  • Customer: Great! Let's loop her into the conversation.

Version 2:

  • Customer: I want a 30% discount or the deal is off.
  • You: We do not offer discounts below the price I've given you.
  • Customer: Oh.

While the "can't" tends to keep the issue open to discussion, the "don't" tends to nips the discussion in the bud.

The same thing is true if you're dealing with an unreasonable request from your boss. Suppose, for example, that your boss asks you to pick up his laundry.

As I explain in my book Business Without the Bullsh*t, if you agree to perform this service even once, you are changing your job description to include picking up your boss's laundry. Here are two versions of the possible conversation:

Version 1:

  • Boss: On your way back from lunch could you swing by the cleaners and pick up my shirts?
  • You: I can't do that.
  • Boss: Why not?
  • You: Well, I'm busy today working on the Acme account.
  • Boss: I need those shirts. It's a priority and it will only take you 5 minutes.
  • You: Oh, OK.

Version 2:

  • Boss: On your way back from lunch could you swing by the cleaners and pick up my shirts?
  • You: I don't pick up laundry.
  • Boss: Oh. Couldn't you do it for me this one time?
  • You: No, I work on accounts like Acme. I don't pick up laundry.

In version 1, the "can't" leaves the door open for pressure; it signals to the boss that you're a pushover. In version 2, the "don't" communicates immediately that you setting a boundary.

Of course, if you true are a hopeless pushover, using "don't" rather than "can't" probably won't have all that much effect.

But if you're like most people, opening with the more powerful word will give you a real advantage if there's a subsequent negotiation and may even cut off the discussion before it takes place.

So, if Yoda were giving your advice about how to handle unreasonable requests, he'd probably say: "Do. Or do not. There is no can't."