The next time you come face to face with a dog wagging its tail, you can make a quick determination on whether to reach out and pet it or step back in deference: check the tail-wag bias. If the wagging tail leans to the dog’s right, you’re safe; if the tail leans to the dog’s left, don’t move.

This tail-wagging bias was documented in a 2007 article in the journal Current Biology by Italian neuroscientist Giorgio Vallortigara and his veterinarian colleagues at the University of Bari. In an experiment, 30 mixed-breed dogs were each placed in a cage equipped with cameras that measured the asymmetrical bias (left or right) of tail wagging while the pooches were exposed to four stimuli: their owner, an unfamiliar human, a cat and an unfamiliar dominant dog. Owners elicited a strong right bias in tail wagging, and unfamiliar humans and the cat triggered a slight right bias. But the unfamiliar dominant dog (a large Belgian Shepherd Malinois) elicited a strong left bias in tail wagging. Why?

According to the researchers, because the left brain controls the right side of the body, and vice versa, the nerve signals cross the midline of the body and cause the dog’s tail to wag more to the right when its left brain is experiencing a positive emotion. This left-right distinction may be explained by the fact that birds, fish and frogs show left-brain/right-brain differences in approach-avoidance behavior, with the left brain associated with positive approach feelings and the right brain associated with negative avoidance feelings. Closer to evolutionary home, when chimpanzees are experiencing negative emotions, they tend to scratch themselves on the left side of their bodies, and left-handed chimps, whose right brain is dominant, tend to be more fearful of novel stimuli than right-handed chimps.

In humans as well, experiments have revealed that the left brain is associated with positive emotions such as love, attachment, bonding and safety. For example, electroencephalogram (EEG) studies of the brains of subjects who report positive emotions or are shown a funny video clip experience an increase in activity in the left frontal cortex, whereas reports of negative emotions and unpleasant video clips coincide with an increase in activity in the right frontal cortex. In addition, brain scans of subjects who are viewing a photograph of a cute baby show increased activity in the same left frontal cortex area; subjects looking at a photograph of a grotesquely deformed baby show increased activity in the same right frontal cortex area. Finally, bombarding the left frontal cortex of the brain with a strong magnetic field elicits a positive mood in human subjects, and the reverse elicits a negative mood.

Why would the brain show such differences in neural networks associated with emotions? Employing evolutionary theory, I would like to suggest that emotions interact with our cognitive thought processes to guide our behaviors toward the goal of survival and reproduction. University of Southern California neuroscientist Antonio R. Damasio, for example, has demonstrated the vital role that emotions play in decision making. At low levels of stimulation, emotions appear to have an advisory role, interacting with the more reason-oriented cortical regions of the brain. At medium levels of stimulation, conflicts can arise between these cortical reason centers and the brain’s deeper and evolutionarily older emotion centers. At high levels of stimulation, emotions can so overrun cortical cognitive processes that people can no longer reason their way to a decision and report feeling “out of control.” But why should we have evolved emotions at all?

Emotions are evolutionary proxies for getting us to act in ways that lead to an increase in reproductive success. If we think of the feeling of hunger as a very basic emotion, for example, a little bit of hunger may be perceived as pleasant, motivating us to seek and find food, whereas too much hunger becomes an unpleasant emotion when it goes unmet. In this homeostatic model, emotions act as a feedback mechanism to alert the brain when the body is out of balance. Positive emotions help us build enduring personal resources, such as problem-solving skills, coordination and social resources. Negative emotions, in contrast, help to protect us. Fear causes us to pull back and retreat from risks. Disgust directs us to push out and expel that which is bad for us. Anger leads us to fight back or to signal displeasure at the violation of a social agreement. Jealousy leads us to guard our mates against intruders in pair-bonded relationships.