Few things are more frustrating than sending an important email message to a business associate or colleague and getting no reply. It’s happened to all of us.

But there are ways to reduce the odds. Behavioral science clearly shows that different appeals trigger different reactions. Unsurprisingly, the subject line of your email is not only the place to start; it’s the key.

In a recent internal experiment, BCG researchers tested different subject lines to see how email recipients would respond. But first, we surveyed participants to see what type of subject “header” they thought would be most likely to trigger a response.

Participants believed the most effective subject headers would be:

• URGENT: Your help needed
• URGENT: [Recipient’s Name]’s expertise needed
• URGENT: [Recipient’s Name]’s expertise needed on a very unusual problem

Then we tested the various approaches. And, as often happens in such experiments, what people thought would work best isn’t what actually worked best. The three most effective subject headers in our test were the following:

• [Recipient’s Name]’s expertise needed
• URGENT: [Recipient’s Name]’s expertise needed on a very unusual problem
• [Recipient’s Name]’s expertise needed on a very unusual problem

There are pros and cons to any approach, of course. Using the URGENT tag if something isn’t urgent can have a “boy who cried wolf” effect and will probably guarantee that the sender’s future emails will be disregarded by many. On the plus side, if the request really is URGENT, recipients of your email are likely to respond more quickly than they would have otherwise.