If you have a high-value site, none of those will work. By making your form different than the masses, you've protected yourself from common attacks. Diversity

If you have a high-value site, none of those will work. By making your form different than the masses, you've protected yourself from common attacks. Diversity is good. However, if you are implementing a sign-up form for an email service there really isn't a great way to detect fraudulent signups without using a captcha. That being said, the end game is that the bots will be able to detect anything a human can. Already, they can detect better than elderly or disabled humans. The path to take in this is to get better at automatically detecting behavior we don't want (wether via human or bot) and to block or delete it.