In their book, The Knowledge Illusion: Why We Never Think Alone, researchers Steven Sloman and Philip Fernbach draw on cognitive science and psychology to put forth a theory of the collective nature of intelligence. In this edited excerpt, they discuss the notion of shared knowledge and the advantages of communal wisdom over individual ideas.

The notion of intelligence has fostered a deep confusion: We think of intelligent acts as performed by individuals even when communities are really responsible. You can see this confusion in how we think about successful companies. Internet start-up entrepreneurs share a mistaken belief with the rest of us: that ideas matter. It is conventional wisdom that the key to a successful start-up is a good idea that can capture a market and produce millions of dollars. That's how Mark Zuckerberg of Facebook and Steve Jobs of Apple did it. Because we assign intelligence to individuals, we give the heroes all the credit by attributing their ideas to them alone. But that's not how it works, according to some of the venture capitalists who fund new start-ups. As one of them, Avin Rabheru, puts it: "Venture capitalists back teams, not ideas."

Consider the view of Y Combinator, one of the leading incubators of early-stage technology start-ups. Their strategy is based on the belief that successful start-ups rarely, if ever, capitalize on their initial idea. Ideas transform. So it's not ideas that matter the most. Far more important than the quality of an idea is the quality of the team. A good team can make a start-up successful because it can discover a good idea by learning how a market works and then do the work to implement the idea. A good team will divide and dis- tribute the labor in a way that takes advantage of individual skills. Y Combinator avoids investing in start-ups that have a single founder not only because a single founder means there's no team to divide up the labor. They avoid single founders for a reason that isn't obvious and yet is fundamental to teamwork: Single founders lack the esprit de corps that prevents individuals from letting their friends down. Teams work harder when things aren't going well because members encourage one another; they do it for the team.

Once you accept that we live in a community of knowledge, it becomes clear that most researchers have been looking in the wrong place for a definition of intelligence. Intelligence is not a property of an individual; it's a property of a team. The person who can solve hard math problems can certainly make a contribution, but so can the person who can manage a group's dynamics or the person who can remember the details of an important encounter. We can't measure intelligence by putting a person alone in a room and giving him or her a test; we can measure it only by evaluating the production of groups that the person is a part of.

How can this be done? What's the right measure of an individual's contribution to group performance? This is not a question that has received a lot of attention. To begin trying to formulate an answer, let's make the simplifying assumption that different individuals do in fact consistently contribute more or less regardless of the group they happen to be in. One approach is to measure the individual's personal contribution across many groups in the same way ice hockey teams measure the contribution of each player, using a plus-minus score. The idea in hockey is that a team will score more goals when a good player is on the ice and the other team will score fewer. So the quality of a player is indicated by a plus-minus score, the number of goals that the player's team scored while the player was on the ice minus the number of goals that were scored against the player's team. One could measure the contribution of a thinker to a group's problem solving in a similar way. When the person is present, how often does the group succeed and how often does the group fail? A person who reliably contributes to group performance and therefore has a high plus-minus score is "intelligent" in a sense that matters. This is potentially a way to reduce collective intelligence to individual contributions in a way that's consistent with the community of knowledge.

A measure like this can be hard to put into practice. One problem is that success and failure are often not as clear as they are in hockey. Is building a widget that wins awards but doesn't sell a success or a failure? Another problem is that if two people tend to work together frequently, then one's success could reflect the contribution of the other one (in the same sense that a man might be perceived as a social success only because his partner is good at making friends).

But the principle holds nevertheless. An executive might seem bright and active, be a great speaker, and provide inspiration all around. But if projects that the executive is a part of tend to fail, then the person may not deserve a big bonus. And when a manager is reviewing employees, it's important not to confuse quick-wittedness and an engaging personality with contribution. The question an employer should ask is whether the projects that the employee is involved in tend to be successful or not relative to other employees.

Every farmer knows that the hard part is getting the field prepared. Inserting seeds and watching them grow is easy. In the case of science and industry, the community prepares the field, yet society tends to give all the credit to the individual who happens to plant a successful seed. Planting a seed does not necessarily require overwhelming intelligence; creating an environment that allows seeds to prosper does. We need to give more credit to the community in science, politics, business, and daily life.

Martin Luther King Jr. was a great man. Perhaps his greatest strength was his ability to inspire people to work together to achieve, against all odds, revolutionary changes in society's perception of race and in the fairness of the law. But to really understand what he accomplished requires looking beyond the man. Instead of treating him as the manifestation of everything great, we should appreciate his role in allowing America to show that it can be great.