"Fixed action pattern" is a term originating from the field of sociobiology that describes animal behavior that is instinctual and indivisible and has a clear end and finish. While this term originally only referred to non-human animals, researchers in human sociobiology, also known as evolutionary psychology, now recognize that humans too have fixed action patterns. The discovery of fixed action patterns in humans relies on research with infants; in fact, many people refer to fixed action patterns as "infant reflexes."
Sucking is one of the best-understood fixed action patterns. This fixed action pattern refers to the sucking behavior infants display when the mouth engages a nipple or nipple-like surface. In evolutionary terms, this fixed action pattern evolved to help infants get the much-needed nutrition they require after childbirth.
One fixed action pattern, coined the Moro Reflex, refers to the reaction of infants' limbs as their heads and trunks become displaced. Biologically, no reason exists for an infant to move her hands as her head and trunk are suddenly moved. However, humans possess this fixed action pattern that commences with the outward flinging of the limbs in response to sudden head/trunk displacement. An evolutionary perspective on the existence of this fixed action pattern holds that the outward-stretched limbs can help alleviate the impact on the head and vital organs in the case of a fall.
The grasping of the hands is a fixed action pattern. Without learning, infants will grasp many objects --- especially those of rope-like, cylindrical shape. An evolutionary explanation for the evolution of such a fixed action pattern holds that this grasping allows infants to hold onto their mother's hair while breast feeding. This capability allows infants to catch themselves from falling from the mother's torso to the ground.
This fixed action pattern regards an infant's knowledge of how to turn his face toward or away from a certain stimuli. In the event that an infant is hungry, upon feeling stimulation upon its cheek, he will face the source of the stimulation. On the other hand, if the infant is not hungry, cheek or mouth stimulus will cause him to turn his head away. This fixed action pattern likely evolved to help an infant in turning his head to or away from the breast to feed or avoid feeding.The cross-culture similarity for us to shake our heads "no" may have risen from this fixed action pattern.