I have created a sign up form for a job finding website:
Should I add a captcha to separate humans from robots when registering for the site? What are its benefits and what effects could this have on the user experience?
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First, you should find out why CAPTCHAs are placed in websites. CAPTCHA is a way to filter the requests from suspicious visitors (robot). So the reason is about "security" itself.
And then about UX concern, "Does it provide a good experience?" The answer is NO. It's very frustrating when I visit a register page at first time, I have to solve the CAPTCHA. Sometimes users are difficult to solve the text, or user mistakenly types the text. It can decreases your "conversion" for your next step.
So, combining the UX and security concern, let follow how Google & Facebook did. They're very user friendly. When login/register, there are no CAPTCHA displayed. But when you're trying to submit the form brutally or you failed to login using password, it shows the CAPTCHA. So the CAPTCHA is showed when an unusual activity detected. It's your security time. The conclusion what they did is: User-friendly: ✓ YES, Secure: ✓ YES
No, you should not use CAPTCHA.
You should focus on technical ways of solving your problems rather than shifting the technical burden onto your users. As a simple example, you are asking for an email address, so you could validate the email address (which you should do anyway) as a substitute for CAPTCHA.
Someone could still write a script to generate many user accounts, but there is no incentive to do this if they don't have an easy way to spam people with user accounts.