Negativity bias is the name for a psychological phenomenon by which humans pay more attention

Attention

Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....

 to and give more weight to negative rather than positive experiences or other kinds of information. This shows up in a number of domains, including:

• When given a piece of positive information and a piece of negative information about a stranger, people's judgment of the stranger will be negative, rather than neutral (assuming the two pieces of information are not severely imbalanced).
• If a person has a good experience and a bad experience close together, they will feel worse than neutral. This is true even if they would independently judge the two experiences to be of similar magnitude.
• Negative information in the simple form of negation has greater impact and creates more attention than similar positive information in the form of affirmation. For example, describing a behavior in an affirmation elicits less attention and cognitive processing than describing the same behavior using a negation. This is related to information processing on negation in cognitive psychology.
• Very often negativity bias is confused with loss aversion, a principle deeply rooted in prospect theory from Kahneman and Tversky. Levy states that negativity bias refers to negative information, whereas loss aversion refers to negative values.
• When put in an environment with a variety of information to pay attention to, people will immediately notice the threats instead of the opportunities or the signals of safety
  Safety
  Safety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
  
  .

The definitive publication on negativity bias in the field of psychology is by Roy Baumeister

Roy Baumeister

Roy F. Baumeister is Francis Eppes Professor of Psychology at Florida State University in Tallahassee, Florida. He is a social psychologist who is known for his work on the self, social rejection, belongingness, sexuality, self-control, self-esteem, self-defeating behaviors, motivation, and...

, Ellen Bratslavsky, Catrin Finkenauer, and Kathleen Vohs
and the phenomenon is often referred to by the paper's title: Bad is Stronger than Good. Another key paper on this bias, developed simultaneously and independently, was published in the same year by Rozin and Royzman.
Prospect theory

Prospect theory

Prospect theory is a theory that describes decisions between alternatives that involve risk i.e. where the probabilities of outcomes are known. The model is descriptive: it tries to model real-life choices, rather than optimal decisions.-Model:...

, developed by Kahneman

Daniel Kahneman

Daniel Kahneman is an Israeli-American psychologist and Nobel laureate. He is notable for his work on the psychology of judgment and decision-making, behavioral economics and hedonic psychology....

 and Tversky

Amos Tversky

Amos Nathan Tversky, was a cognitive and mathematical psychologist, a pioneer of cognitive science, a longtime collaborator of Daniel Kahneman, and a key figure in the discovery of systematic human cognitive bias and handling of risk. Much of his early work concerned the foundations of measurement...

, is an older and independent area of research that produces many of the same results.

Is bad stronger than good?

A more apt description of the theory is that bad is more attention-getting than good. Positive events exert effects in other ways:

• Most people feel mildly positive, most of the time. This suggests that negative events that are sufficiently strong to exert the "bad is stronger" effect may not be that common (see positivity offset
  Positivity offset
  Positivity offset is a psychological term referring to two phenomena: People tend to interpret neutral situations as mildly positive, and most people rate their lives as good, most of the time...
  
  ).
• Positive events can aid recovery from the cardiovascular wear and tear
  Allostasis
  Allostasis is the process of achieving stability, or homeostasis, through physiological or behavioral change. This can be carried out by means of alteration in HPA axis hormones, the autonomic nervous system, cytokines, or a number of other systems, and is generally adaptive in the short term...
  
   caused by the stress response to negative events. This is referred to as the undo effect of positive emotions.
• When positive emotions are experienced regularly, over time, they can have cumulative benefits that are more substantial than the effects of negative emotions. This is referred to as the broaden-and-build
  Broaden-and-build
  The broaden-and-build theory of positive emotions suggests that positive emotions broaden one's awareness and encourage novel, varied, and exploratory thoughts and actions. Over time, this broadened behavioral repertoire builds skills and resources...
  
   effect.
• As people age, the negativity bias is reduced and may even disappear. It is presently unclear exactly what about aging causes this positivity effect
  Positivity effect
  In psychology and cognitive science, the positivity effect is the tendency of people, when evaluating the causes of the behaviors of a person they like or prefer, to attribute the person's inherent disposition as the cause of their positive behaviors and the situations surrounding them as the cause...
  
  .

Nonetheless, when assessing an immediate situation, it does seem that negative information and negative events predominate, and this has significant implications for everything from aesthetics

Aesthetics

Aesthetics is a branch of philosophy dealing with the nature of beauty, art, and taste, and with the creation and appreciation of beauty. It is more scientifically defined as the study of sensory or sensori-emotional values, sometimes called judgments of sentiment and taste...

 to trauma

Psychological trauma

Psychological trauma is a type of damage to the psyche that occurs as a result of a traumatic event...

 recovery to the study of stress

Stress (biology)

Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

 and biochemistry

Biochemistry

Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

.