I remember driving across the bridge from Dartmouth as a child. There were some days when the fog would roll up the harbour and the far shore would disappear, so that the bridge seemed to be arching off into oblivion. I would recall a scene from a film I had once seen: a suspension bridge in an American city waving like a flag, cars struggling to balance on the span until they reached the other side. With bridges, always, there is the chance of falling.

In school, our teachers would ask: If all your friends jumped off the bridge, would you do it too? So I knew, from an early age, that people sometimes hurled themselves over the side.

The old bridge is accessible to pedestrian traffic and is located only a short distance from the poorest area of town — easy walking distance from the bars, the crack houses. When I was a university student, home for the summer, I might come down Brunswick Street in the early hours of the morning and find the bridge closed. It doesn't get reported how many people jump off the bridges, just as larger cities don't publicize the number of people who jump in front of subway cars. It seems, improbably enough, to plant the seed of an idea in people who otherwise might drive or walk the span for years, ignorant of its potential.

Now I'm a psychiatrist, and I hear people say it in the emergency department: I'm going to go jump off the bridge. And I know that, left to their own devices, even the most intoxicated can find it, staggering down North Street in the dark, stumbling, cursing, then climbing unsteadily up on the railing. You can take away their pills, ask their families to hide the guns, but the bridge remains.

Or I see people who have been interrupted in their adventure, picked up on the bridge by the police and brought into the emergency room. Sitting in the waiting room, defiant or contrite, they are like vases caught just as they had begun to tip off a shelf, carrying their near-broken selves around with them like tentative ghosts. So little divides them from the ones who go over the edge: the hesitation, the incomplete impulse, the second thought that might come at that moment, or might not.

I have also encountered half a dozen people who have jumped off the bridge and lived. There is an inevitability that surrounds some of these survivors, like conversations on a cancer ward.

A pathology resident once told me that people who deliberately jump off the bridge leave their eyeglasses neatly folded on the railing behind them. I think I would forget to do that. In the headlong plunge, my glasses would be torn off my face by the force of the acceleration, landing with a soft splash at the spot where I had disappeared only a moment before.

But, in fact, my imagination fails me before that. I can't picture jumping off the bridge. My heart beats faster if I even think of it, as if some impulse might surface that would take me over the side to the moment when it becomes irrevocable, when you are on your way down and there is no going back.

There are impulses bred into us: to turn to a warm breast, or to withdraw sharply from a flame. Not least, a fear of heights. Instinctively, we know that we have to watch our step.

Heights and depths are rich in meaning, a heritage of the time before our ancestors left the trees. There are theological references: high places are associated with the exalted, the divine. Mount Olympus. Man before the Fall, or Satan plummeting from heaven to hell with his legion of angels. Our language reflects this, too: patients who are manic are "high;" the state of clinical despair is "depression," a low place.

We build towers and cathedrals that stretch as high above us as we can make them go. If you sail under the bridge and look up at its prehistoric bones, it is like a monument or a god. If it were a mountain, people would want to climb it. Instead, there is no way up.

The edge of a bridge is one of the few easily accessed places where you can look down from a height instead of up toward it. The feeling is different, less grounded. When you look down, you grow larger, just as everything below you becomes smaller.

It must go against everything body and mind tells you to climb over that rail and jump. I am not the type of person who skydives or hang-glides. I don't crave speed. I don't tempt fate. So I can't imagine that I would enjoy the rush of neurochemicals in the few seconds before hitting the water. Of course, I know that is not what draws people to do it. At least, not entirely.

I spent a recent trip through the Rockies contemplating distances up and down. Up to the turrets on top of the Banff Springs Hotel. Down from the face of Sulphur Mountain. Into the churning waters of Hell's Gate. Wondering how many people have died leaping from these places, and if the psychiatrists who work there have the same worries that I do when I drive home at 3 am, wondering if I should swing by the bridge just to see if anyone I know is out there.

In a lot of children's films, villains fall to their deaths: Snow White, The Lion King, Tarzan, Beauty and the Beast — these are only a few of the movies in which children see evil falling away, its outstretched hand growing smaller and smaller. No one else can be blamed for these deaths. It is the fault of the one wrong step they took, their own error, the dangerous high place they got themselves into.

After the initial loss of balance, falling becomes a passive thing, the killing left up to the combined forces of gravity and water. It can almost seem like an accident, an injury done by the laws of physics. A space forms between the aggression and its object. I wonder if, in the last seconds, those who jump find a relief in thinking that forces beyond their control are at work. Or whether, in that moment, they believe they can fly. Like Icarus, until their wings melt and the feathers come off.