The case against marking form fields as required

To boost response to online forms, Anthony at UX Movement suggests indicating which fields are optional rather than which are required:

Marking required fields enable [sic] users to do the bare minimum to complete your form. They’re going to put more importance on required fields and fill those out first while ignoring the optional ones. Why would they spend time on optional fields if they can fill out what’s required and move on? However, if you use voluntary over-disclosure to your advantage and mark optional fields only, users won’t feel the need to take shortcuts.

Worth a test.