Humans have attention spans like goldfish. That’s the current thinking in large parts of the web design industry. Whilst we’re not entirely sold on that idea (it seems a bit harsh!) there is certainly some merit to the thinking behind it.
Internet users are very fussy about the time they spend online, and your website has a very short period of time in which to convince them to stay longer and hear what you have to say. Research by the Nielsen Normal Group suggests that you may have as little as 10-20 seconds to grab your users’ attention.