So runs the common wisdom and it has done for a few years. If my memory serves me well, this notion was first widely accepted back in the 1990s (thank you Mr Nielsen).
But I did some testing a while ago and something strange happened. After dealing with the usual university-aged, I-can't-stop-fiddling-with-the-mouse younger people raced through the screens as if their lives depended upon it, the next participant was an accountant nearing retirement. He was well-established and well-respected in his town and used the web a lot to access information about his customers. He said (and I had every reason to believe him) that he had used the web for many years, probably longer than myself so keen was he on technology's ability to improve work.
But then the strange thing happened. I told him what I wanted him to do and opened up the first screen. After sitting there with poised for action, he took the screen in a glance and then sat back. Right hand off the mouse, left hand off the keyboard, arms folded. And he carefully read - repeat, read - his way through the content shown on screen. When finished, he then reached out for the mouse and slowly but carefully moved the screen to view the remaining couple of lines. Once he had set up his screen, his hand was straight off the mouse again. And then he read, slowly and carefully again. He asked a question about the grammar in one sentence, a perfectly valid one too.
And after that, I had a few more users reading carefully instead of scanning quickly. I quickly introduced a new element into the study. At the end, I asked them questions about the first page's content and found recall to be quite good. These readers really were reading and taking the information in.
So do users scan?
Yes and no. Some do and some don't. But even those who scan lots will sometimes seem to slow down and read; whereas others who read will skip through text.
But what lies at the bottom of all this? What makes us read or scan?
I have no formal testing behind this but I would suggest three things:
1) User's disposition: some, like our esteemed accountant, seemed naturally disposed towards reading. Others want to hurry up.
2) Task importance: is this just finding out a trivial piece of information? Or is it making a large financial transaction? The more important a user thinks their task is, the more likely it is that they will read rather than scan.
3) Relevance: using superficial cues (like the title or summary), users will make quick conclusions about the relevance of text to what a user wants to do. If text seems to be completely irrelevant, they are more likely to ignore it.
These are only from observation so treat them more as hypothesis generation than anything firm. However, I do think they are good; but evidence trumps all.
And how does this affect design?
1) Remember than not all users scan. Some have strategies to read thoroughly through web text content.
2) If a task is important and can have serious consequences, tell users. Conversely, if it's trivial and can easily be reversed, tell them that too (but then, where do you stop?)
3) Help users to make great relevant decisions up quickly. Try to use very descriptive titles rather than ones which might mislead.
Either way, it's wrong to say, "Users only scan".