Emergent properties are ones that obtain at one level while being absent at the other.
A formal way to relate multi-level descriptions of a phenomenon is given by the theory of model aggregation. The concept of an emergent property of a system falls out naturally from considerations of model aggregation, and is distinguished from simple resultant properties. Two distinct types of emergent properties are identified. Examples of emergent properties in social systems are given. Rob Axtell is Co-Founder of the Center on Social and Economic Dynamics at the Brookings Institution.