From a biological perspective, culture may be broadly defined as shared variation in behavior that is generated and maintained by social learning—through imitation or teaching, for example. Social learning in animals is often difficult to demonstrate directly. But the presence of culture can be established by observation and deduction: when behavioral differences exist that cannot be accounted for by genetic or environmental factors, cultural transmission must be occurring. Critics respond that it is often difficult to rule out hypotheses that genes or learned individual responses to differing environments are responsible for behavioral patterns. Often implicit in this argument is the notion that social learning, considered a more complex and more cognitively demanding phenomenon than individual learning, should be invoked only as an explanation of last resort.