Of all the -isms, the hardest to swallow might be determinism — the idea that our actions are beyond our control. Jonah Berger, a marketing professor at the University of Pennsylvania’s Wharton School, puts a fresh dent in the notion of free will in his new book, “Invisible Influence: The Hidden ­Forces that Shape Behavior,’’ in which he argues — with remarkable specificity — that “Ninety-nine-point-nine percent of all decisions are shaped by others.’’

Reflexively, we are told, we mimic the behaviors of people around us. We buy a certain car because it suggests the right kind of identity or because Bob in accounting just bought one or simply because we’ve seen a lot of them around and “the more people see something, the more they like it.’’ Similar dynamics are involved in everything from the books we read to the people we date.

Berger cites a number of social experiments to back up his claims. In one, eight individual websites invited users to download songs from a list of preselected bands; the most popular choice varied wildly from site to site, according to which band had been lucky enough to get the early clicks. Nothing draws a crowd like a crowd.

Even when we rebel, we do so in predictable ways. Some visitors to those music sites would have willfully clicked the band at the bottom of the list, thereby following the crowd that prides itself on not following the crowd. Conformity is a predisposition. Individualism is an illusion. There is no such thing as a special snowflake.

It’s comforting to think that such principles don’t apply to critical decisions — as in, say, a presidential election. Right?

Ideas spoke with Berger by phone from his home in Philadelphia. Below is an edited excerpt.

IDEAS: Why are people are as likely to be influenced by others when making huge decisions as they are when making small ones?

BERGER: You might think, “Oh, when it’s a really important decision, that’s when we turn on our thinking and go our own way.’’ But that’s not what happens. We figure out what career to pursue based on what others have pursued. We decide on what home to buy based on how many days it’s been on the open market, because that’s a signal of whether other people like it or not.

The bottom line is: People follow the pack.

IDEAS: Why do they?

BERGER: Influence is like a magnet: Sometimes it attracts, sometimes it repels. Women might be less interested in careers in technology because of what it signals about them, the association with geeky males. Then there are those who avoid a certain brand because of the people it’s associated with.

IDEAS: You can understand not buying a kind of bag because you saw someone on “Jersey Shore’’ with one, but does this dynamic really carry over into something as important as politics?

BERGER: I was working on a project recently, trying to get clean energy to catch on among conservatives. On the face of it, conservatives should love clean energy: It’s cheaper; it reduces reliance on oil, so is better for national security. Yet the support isn’t there. As for why, one politician put if very nicely: “If this is something Al Gore stands for, it’s probably not for me.’’ So it’s not just about what clean energy does, but what it means.

IDEAS: You write about the little green frog that croaks like a big green frog to fool its rivals. Humans behave in similar ways — we fib to make others think better of us. Which may help explain why polls can be so unreliable.

BERGER: Yes. Sometimes, we don’t signal our true identity but an ideal identity. There was a concern a few years ago that people who said they supported Barack Obama in exit polls did so because they didn’t want to appear racist. This wasn’t only the case with people who were actual racists, but those who honestly had different political views.

IDEAS: Worryingly, these fibs can have a real effect on who wins an election.

BERGER: It’s the snowball effect: Polls affect how people vote early on, which affects how other people vote. You end up in a cycle, where things go right when they could just as easily have gone left. So the tendency to follow others can affect not only our individual decisions, but the ones we make as a country.

IDEAS: An even flimsier motivation is “familiarity leads to liking’’ — does that theory help explain the rise of the highly visible Donald Trump, even when much of the coverage about him is so negative?

BERGER: I think this is more of a case of him reinforcing his message. There’s this notion, particularly in American culture, of the rugged individual, the myth of the outsider. So with Trump, the idea is that, unlike other politicians, he just says what he feels, he’s not part of the Washington culture. Even when he says things that people don’t believe, there’s still this idea that, hey, at least he’s speaking his mind.

IDEAS: You argue that a dissenting voice, even one that’s not particularly palatable, can be a valuable thing in itself.

BERGER: Having someone disagree, even if their view may not be the same as yours, can free you to express your dissenting opinion. People love to talk about the wisdom of crowds, but crowds are only wise if everyone is chipping in. If everyone is just following everyone else, it becomes a problem.

IDEAS: Which brings us to differentiation, the urge to go our own way. Your suggest that even rebellion has a conformity to it.

BERGER: We do tend to be different in predictable ways. We might see ourselves as acting in a way that’s unique, when we’re just being similar to a smaller group of people. But this isn’t necessarily a bad thing. We have this notion that being different is the right way to be. What it comes down to is being optimally distinct, being similar enough that we don’t feel alone and different enough that we feel unique. This are the two key human motivations that dictate the way we behave.

IDEAS: What people really crave, in the end, is a blend of the familiar and the novel, which leads you to write: “The next big thing often has elements of the last big thing.’’ So, by this measure, Hillary Clinton is going to win the election.

BERGER: Ha, ha! I’m not sure I have that kind of predictive power.

Chris Wright is a London-based editor and writer.