Here's an interview with Michael Mauboussin, Chairman of the Board of the Santa Fe Institute, an organization which specializes in the multidisciplinary study of complex adaptive systems.
Complex adaptive systems are comprised of heterogeneous agents (e.g., ants in an ant colony, neurons in your brain, investors in a market) which interact leading to "emergence". Emergence produces a global system with properties and characteristics that cannot be deduced solely by looking at the heterogeneous agents. As Mr. Mauboussin says, reductionism doesn't work.
In other words, resulting behaviors of these systems are not always what you might expect.
Mauboussin on the Santa Fe Institute and Complex Adaptive Systems