A while ago, I read a book called Chaos by James Gleik, which presented a whole new way of looking at the world. We are used to thinking of science being a case of cause and effect: something happens, and because of that something else, and so on. Everything that exists is there because of something that happened before it, and so on back to whatever the first cause was. One consequence of this, of course, is that what exists now could not have happened any other way, that everything is a result of inexorable laws. That I am writing this now was decided at the Big Bang, and it all fell into place billions of years later in the form of me and this computer, and you reading it. Everyone, whether saint or serial killer, could not have done other than they did, because a molecule 3 billion years ago took a particular path and not another.

The Chaos book did not deny any of this. It does, however, look at those causes. The “Butterfly Effect” refers to the idea that small causes can have vast, unpredictable consequences. The term, coined by meteorologist Edward Lorenz in the 1960s, was born from a striking observation: when he entered slightly rounded data into a weather model—changing a number from 0.506127 to 0.506—the simulated weather diverged wildly. It was as if, in metaphorical terms, a butterfly flapping its wings in Brazil triggered a storm in China.

The Earth’s atmosphere is a swirling soup of temperature gradients, moisture levels, barometric pressures, oceanic patterns, and atmospheric particles—from volcanic ash to insect wings. One microscopic swirl of air, unmeasured and unnoticed, can blossom into a thunderstorm or break apart a cyclone. This doesn’t mean weather is magic—it obeys physical laws—but its sensitivity makes the system effectively unpredictable beyond a short window. The reason is simple: we can never know all the variables.

But, you might say, human society is surely not subject to these laws. We have “free will” whatever that means. There was a movie called Sliding Doors, which showed Gwyneth Paltrow both missing and catching a train, and the life-altering consequences of each alternative. But that action depended on something happening which was pre-determined. There is a quantum theory called the Many-Worlds Interpretation (MWI) which says that at every point the universe splits into multiple versions, one for every possible outcome. But can there be another possible outcome than the one which actually happened?

Consider a well- known event. Most people in the world were aware of the happenings of September 11, 2001, either at the time or soon afterwards. What is not as obvious is that on that day, Osama Bin Laden (or whoever was responsible) totally changed the human gene pool forever after. How? After that event, the daily behavior of billions of people shifted, even if only subtly. Some stayed home. Some moved cities. Some changed jobs, or fell into depression, or reached for connection. Some couples broke up. Others drew closer. And for any man who conceived a child after that day, the act of conception—the exact sperm cell that reached the egg—was subject to a butterfly chain of microscopic events. Sperm compete in a race governed by chance and timing. One heartbeat’s delay, one moment of hesitation or passion, and a different sperm wins. The result: a completely different child. Had 9/11 not happened, no one conceived after that time would ever have existed. Different ones would have taken their place, or none at all. Biologically, historically, and even philosophically, small shifts create vast consequences. That attack created everyone in the next generation—not just in the USA, but across the globe.

Of course, this applies not only to major events like 9/11, but even the most minor. For example, your grandfather was walking down the street sometime in the last century. His attention was distracted momentarily by a reflection in a window, so he turned his head slightly. In consequence, his body shifted, and the hundreds of millions of sperm he was carrying at the time moved just a bit. So when he and your grandmother created your mother later that night, it really was her. Had that reflection not impinged momentarily upon his retina, another sperm would have won the race, and it would not have been your mother at all, and you simply never would have been. And what caused that reflection? Obviously something totally unrelated—a stray dog, a passing car, the sun peeking out of a cloud—which just happened to play a crucial part in creating you, along with the countless other things that did and didn’t happen to him, and her, and every one of their ancestors, on that and every other day up to then.

I don’t know what the lesson is in all this. Maybe this: