I'm probably not the first with this idea but I don't know the right terms to find an article/topic on this.

My idea is to have one central login/registration place starting with only an e-mail field, then when a user has filled in their e-mail, different options appear based on whether or not the e-mail is already registered.

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I'm curious what would be the downside(s) of this approach.

  • If you didn't use e-mail addresses as a user name (which has various downsides, anyway), the privacy issues described in the answers so far would not be that severe. – O. R. Mapper Mar 2 '18 at 12:25

Many users consider it a violation of privacy to publicly reveal in this manner that they have an account with your service.

Also, passwords are bad for lots of reasons. I know of two sites that simply don't use passwords:

and Slack encourages users to login via email instead of typing a password. This answer on a Security.SE question also explains the above concept from a slightly more technical perspective.

However, if you want to ignore all that advice about protecting your users, see this question on UX.SE about combining login and signup forms. It will answer your question if you insist on using passwords.

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    I agree with the security aspects. Unfortunately, the described login process via e-mail is awful usability. Moreover, if I access a service on a machine other than my own one, it substitutes "open service website -> type e-mail -> type password -> login to service" with "open service website -> type e-mail -> open e-mail websiite -> type e-mail -> type password -> read e-mail -> login to service", which doesn't seem all that much more secure to me. – O. R. Mapper Mar 2 '18 at 12:15

Someone trying to hack that persons email, would be my guess. You've confirmed that you know that email address exists in your database.

That's not to say some mainstream sites don't implement something like this and it works rather well, experience wise.

Only other downside I can see at the moment how you implement the change in the form, it would need to be seamless and not move between pages, in my eyes, and how you cater for communication issues in the request to see whether the address exists.

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  • Regarding implementation, many sites are now separating username and password entries onto different pages. i.e. You enter your username/email and click "Sign In" or "Continue" or whatever, and it takes you to a page asking for a password. Google does it, and I think Amazon AWS as well. – Dwev Aug 3 '17 at 10:21
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    Doesn't mean it is good UX. – DarrylGodden Aug 3 '17 at 10:24
  • No, it doesn't, but it can be a way of implementing a welcome on a different page, prior to the actual login, or redirecting to a registration page if they aren't already a user. – Dwev Aug 3 '17 at 11:26

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