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In the US, famous faces give graduating students stirring send offs. Simon Usborne on the secrets of the commencement speech

Madam reader, distinguished guests and honoured friends... It is a privilege to stand before you and take you on a journey along a road strewn with uplifting aphorisms, positive-thinking platitudes and parables. For they are the stock in trade of the commencement speaker, the master of an all-American art exhibited at universities across the land of the free and the unfailingly optimistic.

Each year around now, US colleges fight to enlist the biggest names in politics, business, and entertainment to speak at their graduation ceremonies. The class of 2012 has so far received lessons in life from President Barack Obama ("Persevere, nothing worthwhile is easy"); Oprah Winfrey ("Be in the driver's seat of your own life because if you're not, life will drive you"); and Alice Cooper ("Apparently, chickens don't fly so much as they plummet").

Their brief is simple: inspire and entertain. Some commencement speeches then veer towards the pompous and schlocky, like American football huddles for smart people. Others are stand-up routines delivered in gowns. The best are self-deprecating, funny, and transporting. They all form a peculiarly American educational tradition that, like proms and yearbooks, evokes envy and intrigue in other countries.

But while commencement speeches are not a feature of British life, the words they produce increasingly carry weight beyond the ranks of the typically hungover students to which they are addressed. When Sheryl Sandberg, the chief operating officer at Facebook, told aspiring fellow entrepreneurs at Harvard Business School recently that it was OK for women to cry at work, it triggered a media debate across the world.

Sandberg has form, telling the all-female Barnard College last year: "You are the promise for a more equal world... I truly believe that only when we get real equality in our governments, in our businesses, in our companies and our universities, will we start to solve this generation's central moral problem, which is gender equality."

Hollywood supplies glamour. Meryl Streep is a regular, this year addressing students at the University of New Hampshire. Robert de Niro was on hand at the private Bates College in Maine, where the actor and school-leaver also received an honorary degree. "Leaving school when I did it was an advantage," he said. "I saved nearly $6,000 by not having to pay tuition and expenses for four years of education. I feel a little foolish, because if I had waited until now not to go to college, I could have saved around a quarter of a million."

Making fun of very privileged young people also helps to broaden the appeal of the commencement speech. Aaron Sorkin, screenwriter of The West Wing and The Social Network, told students at Syracuse University in New York: "Make no mistake about it, you are dumb. You're a group of incredibly well-educated dumb people... You're barely functional."

He added, with a rare but welcome note of pessimism: "There are some screw-ups headed your way. I wish I could tell you that there was a trick to avoiding the screw-ups, but the screw-ups, they're a-coming for ya. It's a combination of life being unpredictable, and you being super dumb."

To watch or read these speeches and other famous examples go to Ind.pn/ComSpeech