Last week it was Rudy Giuliani's cell phone. This week, it's Hillary Clinton's cackle.

The media has been all atwitter about the Democratic presidential candidate's propensity lately to laugh a lot -- on news talk shows, at impromptu press conferences and before voters. Not just a titter or a giggle, either, but a great big guffaw of a belly laugh -- sometimes at questions that, at first blush, don't seem to be all that side-splitting but sometimes downright hostile.

What's going on here?

Some critics think Sen. Clinton is trying to warm up an image often criticized as cold and humorless -- while others say she's merely gotten more comfortable about showing her real self on the campaign trail.

University of Maryland professor Robert Provine, author of "Laughter: A Scientific Inquiry," thinks it is probably the latter, since laughter is usually "an honest signal. It's hard to lie with laughter."

Most everyday laughter isn't in response to jokes, he added, but rather an attempt at social interaction. People may laugh out of nervousness, anxiety, skepticism, surprise or an eagerness to please -- but this particular human vocalization is fundamentally a social act.

"You don't have too many cases of people sitting alone in a room laughing uproariously," he said (solo viewers of "The Office" excepted).

But if Sen. Clinton's laughter is a calculated move to deflect tough questions or win over voters, her handlers better move on to Plan B, he said.

"If it was working better, people would be talking about charisma in reference to Hillary instead of 'bizarre cackling sound.' "