Writing in all capital letters is like shouting in print.

That's the lesson from Antoinette LaFarge, a professor of digital media at the University of California at Irvine .

According to a recent online survey , 40 percent of Congress.org users say they "sometimes" or "often" use all capital letters when writing Congress.

But in an interview with Congress.org, LaFarge says it can make your writing less effective.

After teaching typography and graphic design for the past 10 years, she has formed a few theories about why people write in all caps and how readers react to it.

Congress.org: Where did all caps come from?

LaFarge: The English alphabet comes primarily from the Greek and Roman languages.

In the classical era, all caps were used for stone engraving on monuments. The letters were designed to be legible at a distance — the same way people often use very large type or uppercase letters now on billboards.

Lowercase letters developed later, when people began using various early forms of paper. Because of the way handwriting works, the letter forms started to mutate.

After the development of typesetting, you had a mix of both upper- and lowercase and refinements in the use of type. But all caps really came on strong during the computer era.

When people began writing in online forums, you started to see more use of all capital letters. By the mid-1990s, you started to see people comparing it to shouting.

Congress.org: What made writing in all caps so popular with computers?

LaFarge: One of the problems that people have in writing is showing emphasis.

In print, graphic designers had a lot of tricks to do this such as changing the size of a font, the spacing between letters, color, italics and bold type. All of that stuff was pretty much unavailable in the early days of the Internet. You just had plain vanilla type.

So how do you create emphasis? One way was to surround words with asterisks, which was supposed to be the equivalent of bold. But that was just clumsy.

So people reached for the first thing they could find, which was all caps.