<select> based CAPTCHA ?

What do you think about this ?

Interesting approach and pretty universal assuming people can count. Only thing is, wouldn't it be better to have the text above the selection box telling them which one to select, then just have the list of options below, eg:


Please select the fifth option from the list below.

[LISTBOX]
Option 1
Option 2
Option 3
Option 4
Option 5
[/LISTBOX]

Would still be hard for bots and actually possibly harder as in HTML the instruction and the actual box would be further apart. Can't imagine anyone programming a bot specifically to work this out anyway.

Of course if you really wanted to get clever, you could use something like animal names in there or well anything else, but I guess that requires a little more intelligence from the public and I never too convinced that exists...

Trev

This seems similar to the practice of having the user respond to a mathematical operation, for example "Human verification: What is 2 + 4? <input>"

Is there a reason this would be more or less efficient?

I don't know about efficient, but it's not something I've seen used before. Which means it would be impervious to spam bots that aren't specifically designed with a certain site in mind, at least for a little while.

I use a similar thing on my contact form, basically I have a reasons <select> element with random options that say "I want to send you spam" mixed in with the real answers. I drop submissions that don't use one of the real options. Since I started using it my spam almost vanished, at least for the time being.