Causality, the relationship between causes and effects, has ­always fascinated me.

It’s the fundamental bedrock of the natural sciences, and the place from which Aristotle started when answering the question, “Why?”

And it’s a deceptively simple theory.

Most of the time, cause and effect are easy to see—you throw a baseball at a window and you produce shattered glass. But the part that captures my imagination is what’s known as the butterfly effect, a tenet of chaos theory that maps the indirect effects of a seemingly unrelated cause.

Basically, as the saying goes, a butterfly flaps its wings in China and a storm forms in New England.

Of course, like many theories, the idea doesn’t bear close scrutiny in a hyperliteral sense: How could something so small, a butterfly’s wings, cause a storm half a world away? And why isn’t The Weather Channel on top of this?

But think of a ball on top of a hill—which way it rolls down the hill depends on a lot of little variables, such as a small gust of wind, or a pebble in its way, or slight differences in where it’s placed.

The point is, little things can mean a lot. And when you think about the consequences of all the little things you do, of all the split-second decisions and reactions that shape the course of your life, it’s easy to come to this conclusion: There are no little things.

In the multifamily world, it’s relatively easy to map out the direct effect a new apartment community has on its broader neighborhood. Construction jobs are created, and in the process, money is pumped into the local economy. This fact is often a selling point for ­developers when trying to curry civic support for a new project.

Yet, as it turns out, developers and owners have a much greater story to tell than they’ve been telling.

Construction is only part of the picture, and a little part at that: The economic impact of operations and resident spending is far bigger and farther-reaching. Until now, nobody had attempted to add it all up, to quantify the gross domestic product of the multifamily ecosystem.

But as you’ll see in our cover story this new research from the National Multi Housing Council and the National Apartment Association isn’t merely an academic exercise. It’s news you can use to win over skeptical locales, a series of must-have bullet points and metrics that illustrate our industry’s importance to not just our neighborhoods, but, cumulatively, to the national economy.

In short, all of us who work in the multifamily ­industry are part of a cause that has an amazing effect, a trillion-dollar business with a local flavor.

So a single event, a new apartment community, sets multiple wheels in motion. In economics, this principle is known as a multiplier effect, another form of causality. And as you’ll see in “Multiplier Effect,” this principle can apply to social media, as well: Done correctly, you can set off a chain reaction on the Web that accrues to your bottom line.

Perhaps there’s no better illustration that little things can mean a lot than ancillary income, as you’ll see in our feature.

All of this reminds me of the vacation fund I formed during my first job out of college. At the end of each day, I’d take all the coins out of my pocket and plunk them into a series of 2-liter soda bottles. After a few years, I filled a dozen bottles and had saved about $700, which I used to go to Alaska.

As the legendary basketball coach John Wooden once said, “Little things make big things happen.” MFE