Most of us are presented with millions of sensory stimuli a day. How do we know what to attend to and what to ignore? What tells us that it's okay not to notice each and every leaf on each and every tree that we pass, but important to attend to the dip in the sidewalk in front of us? Though perception is different for each person, we each attend to the stimuli that are meaningful in our individual worlds. Selection is the process by which we attend to some stimuli in our environment and not others. Because we cannot possibly attend to all of the stimuli we are presented with, our brains have an amazing unconscious capacity to pick and choose what's important and what's not. 

The Influence of Motives

Motivation has an enormous impact on the perceptions people form about the world. A simple example comes from a short-term drive, like hunger: the smell of cooking food will catch the attention of a person who hasn't eaten for several hours, while a person who is full might not attend to that detail. Long-term motivations also influence what stimuli we attend to. For example, an art historian who has spent many years looking at visual art might be more likely to pay attention to the detailed carvings on the outside of a building; an architect might be more likely to notice the structure of the columns supporting the building.

Perceptual expectancy, also called perceptual set, is a predisposition to perceive things in a certain way based on expectations and assumptions about the world. A simple demonstration of perceptual expectancy involves very brief presentations of non-words such as "sael." Subjects who were told to expect words about animals read it as "seal," but others who were expecting boat-related words read it as "sail."

Emotional drives can also influence the selective attention humans pay to stimuli. Some examples of this phenomenon are:

• Selective retention: recalling only what reinforces your beliefs, values, and expectations. For example, if you are a fan of a particular basketball team, you are more likely to remember statistics about that team than other teams that you don't care about. 
• Selective perception: the tendency to perceive what you want to. To continue the basketball team example, you might be more likely to perceive a referee who makes a call against your favorite team as being wrong because you want to believe that your team is perfect.
• Selective exposure: you select what you want to expose yourself to based on your beliefs, values, and expectations. For example, you might associate more with people who are also fans of your favorite basketball team, thus limiting your exposure to other stimuli. This is commonly seen in individuals who associate with a political party or religion: they tend to spend time with others who reinforce their beliefs.

The Cocktail Party Effect

Selective attention shows up across all ages. Babies begin to turn their heads toward a sound that is familiar to them, such as their parents’ voices. This shows that infants selectively attend to specific stimuli in their environment. Their accuracy in noticing these physical differences amid background noise improves over time.

Some examples of messages that catch people’s attention include personal names and taboo words. The ability to selectively attend to one’s own name has been found in infants as young as 5 months of age and appears to be fully developed by 13 months. This is known as the "cocktail party effect." (This term can also be used generally to describe the ability of people to attend to one conversation while tuning out others.)

Cocktail Party Effect

One will selectively attend to their name being spoken in a crowded room, even if they were not listening for it to begin with.

The Influence of Stimulus Intensity

A stimulus that is particularly intense, like a bright light or bright color, a loud sound, a strong odor, a spicy taste, or a painful contact, is most likely to catch your attention. Evolutionary psychologists theorize that we selectively attend to these kinds of stimuli for survival purposes. Humans who could attend closely to these stimuli were more likely to survive than their counterparts, since some intense stimuli (like pain, powerful smells, or loud noises) can indicate danger. More than half the brain is devoted to processing sensory information, and the brain itself consumes roughly one-fourth of one's metabolic resources, so the senses must provide exceptional benefits to fitness.