Many forms don’t specify which radio button is selected by default, in order to give the visitor the option of not answering the question. This is error-prone, and has the potential to cause problems for visitors.
If you want to give visitors the option of not answering a question, this should be included as an explicit option (selected by default). This gives users the opportunity to change their minds (without this option, if they had made a choice and then decided not to answer the question, there would be no way of unchecking the option). Also, some browsers may automatically select an option if none is selected by default (this was the behaviour specified by RFC 1866).
For more information on the use of forms, and other usability issues, try Jakob Nielsen’s excellent book, Designing Web Usability:
- Designing Web Usability at Amazon.com
- Designing Web Usability at Amazon.co.uk (for British readers)