The mapping between genes and behavior is made even more complex by the fact that few if any neural circuits operate entirely autonomously. Except perhaps in the case of reflexes, most behaviors are the product of multiple interacting systems. In a complex animal like a mammal or a bird, virtually every action depends on a coming together of systems for perception, attention, motivation, and so forth. Whether or not a pigeon pecks a lever to get a pellet depends on whether it is hungry, whether it is tired, whether there is anything else more interesting around, and so forth. Furthermore, even within a single system, genes rarely participate directly “on-line,” in part because they are just too slow. Genes do seem to play an active, major role in “off-line” processing, such as consolidation of long-term memory—which can even happen during sleep—but when it comes to rapid on-line decision-making, genes, which work on a time scale of seconds or minutes, turn over the reins to neurons, which act on a scale of hundredths of a second. The chief contribution of genes comes in advance, in laying down and adjusting neural circuitry, not in the moment-by-moment running of the nervous system. Genes build neural structures—not behavior.