The moments that shape our lives are much more likely to be small and personal, rather than huge national or international events, according to a leading professor.
A famous defining moment - where were you?
Speaking on BBC World Service's The World Today programme as part of its Defining Moments series, Frank Furedi, Professor of Sociology at the University of Kent, said that often people don't notice that a key point has occurred until some time after the event.
"You get a sense that these people have had an experience that caught them unaware," Professor Furedi told The World Today.
"Sometimes they could be really large events, like being forced to confront death - but other times it's a fairly minor episode.
"Desmond Tutu, for example, is caught unaware by the fact that he meets someone who treats his family in a very different way than he has been used to."
The Defining Moments series features personal reflections from the likes of South African anti-apartheid campaigner Desmond Tutu, former Opec boss Sheikh Yamani, and journalist Bob Woodward, who exposed the Watergate scandal that brought down US President Richard Nixon.
"The key element here is that it is always something that goes against the grain of your life, something that you've never really experienced before, that opens your eyes to new possibilities - and life is never the same after that," Professor Furedi said.
"Sometimes very minor personal events, that you don't notice at the time, sneak up on you and a week later, or a month or a year later, you realise you're never the same person again."
He added that often the major events that people remember - such as the assassination of US President John Kennedy - actually have only a limited impact on their daily life.
"The really big events are often far too distant from your experience to have that kind of personal, immediate impact," Professor Furedi explained.
"Everybody has got a story about when JFK died, but it is pretty much a story that it is part of culture - you're meant to remember that event, it's something that's expected of you.
"These discreet, personal, intimate moments are your own, and that's something you internalise far more than these big events as represented by JFK."
And he added that even if you are unable to place what your defining moment is, it does not mean it has not happened.
"All of us have experiences that may not be a defining moment but certainly will have influenced you in ways that are very difficult to express verbally," he said.
"If you think long and hard, it may well be the case that as you get a bit older you realise that - 'yes, that woman who told me something very interesting did change the course of my life, even though at the time I didn't take very much notice of it'."
Defining Moments will run until 23 July on BBC World Service's World Today programme. You can also read people's recollections on BBC News Online.