Usability is about making making web sites useful, easy, and likeable.

How about persuasive?

Enter Captology: computers as persuasive technology.

Persuasion includes (as you can see from the SPTL figure on the right):

• behavior change
• attitude change
• motivation
• change in worldview
• compliance.

The first deep resource is the Stanford Persuasive Technology Lab. Browse through the key concepts and examples. Or visit a spinoff, the Stanford Guidelines for Web Credibility.

Principle of Easy Verifiability: Credibility perceptions will be enhanced if a Web site makes it easy for users to check outside sources to verify the accuracy of site content. 

If you're interested, pick up a copy of B.J. Fogg's Persuasive Technology: Using Computers to Change What We Think and Do. The appendix alone will change the way you design everything from weblogs and surveys to ERP apps, job sites, and computer games. 42 principles backed by research at Stanford and MIT. For example:

Principle of Tunneling: Using computing technology to guide users through a process or experience provides opportunities to persuade along the way.

The book provides more context, cases, and implementation notes.

You can see a new discipline emerging. One that may explain why some sites win and others lose.

For example, both Kerry for President and Dean for President use meetup.com to build grassroots support. Dean overwhelmed the meetup venues (they needed 3 just in my thirds of Alameda county) while Kerry had 4 people show up in the single event in 4 counties (Alameda, Contra Costa, Marin, and San Francisco). It's not the candidates, even in the polls. It's how their campaigns used their web sites to persuade people to participate, to convene, to volunteer.

Principle of Reduction: Using computing technology to reduce complex behavior to simple tasks increases the benefit/cost ratio of the behavior and influences users to perform the behavior.

Why is persuasion important?

First, to serve your visitors' goals, it helps to guide and direct them. A usability hallmark. Persuading them to follow the trail that accomplishes their goal is in their interest.

Second, to accomplish your site's missions. Your site may inform and entertain, but persuasion is crucial for many personal and institutional purposes.

Politics, Religion, and other places where the marketplace of ideas matters.

Social services, healthcare, education, where helping people help themselves calls for change.

And the worlds of trade, commerce, employment, leadership, diplomacy, where mutual persuasion leads to results everyone can tolerate.

New skills sure to become fundamental for every experience designer and marketer. 

Principle of Authority: Computing technology that asumes roles of authority will have enhanced powers of persuasion.

Trust me.