Wouldn't it depend entirely on which is more common in your form? If it's a medical form and 95% of the fields are required, mark the optional ones. If it's a s

Wouldn't it depend entirely on which is more common in your form? If it's a medical form and 95% of the fields are required, mark the optional ones. If it's a social network sign-up page and most of the fields are optional, mark the required ones. I don't know what kind of slope-browed sadist wouldn't mark anything at all.