Neuroticism
Neuroticism is a fundamental personality trait in the study of psychology. It can be defined as an enduring tendency to experience negative emotional states. Individuals who score high on neuroticism are more likely than the average to experience such feelings as anxiety, anger, guilt, and clinical depression. They respond more poorly to environmental stress, and are more likely to interpret ordinary situations as threatening, and minor frustrations as hopelessly difficult. They are often self-conscious and shy, and they may have trouble controlling urges and delaying gratification. Neuroticism is related to emotional intelligence, which involves emotional regulation, motivation, and interpersonal skills. It is also considered to be a predisposition for traditional neuroses, such as phobias and other anxiety disorders.

# Emotional stability
On the opposite end of the spectrum, individuals who score low in neuroticism are more emotionally stable and less reactive to stress. They tend to be calm, even tempered, and less likely to feel tense or rattled. Although they are low in negative emotion, they are not necessarily high on positive emotion. That is an element of the independent trait of extraversion. Neurotic extraverts, for example, would experience high levels of both positive and negative emotional states, a kind of "emotional roller coaster." Individuals who score low on neuroticism (particularly those who are also high on extraversion) generally report more happiness and satisfaction with their lives.

# Measurement
Neuroticism is typically viewed as a continuous trait, rather than a distinct type of person. People vary in their level of neuroticism, with a small minority of individuals scoring extremely high or extremely low on the dimension. Because most people cluster around the average, neuroticism test scores approximate a normal distribution, given a large enough sample of people. Neuroticism is one of the most studied personality traits in psychology, and this has resulted in a wealth of data and statistical analysis. It is measured on the EPQ, the NEO PI-R, and other personality inventories.

# Physiology
Neuroticism appears to be related to physiological differences in the brain. Hans Eysenck theorized that neuroticism is a function of activity in the limbic system, and research suggests that people who score highly on measures of neuroticism have a more reactive sympathetic nervous system, and are more sensitive to environmental stimulation. 
Behavioral genetics researchers have found that a substantial portion of the variability on measures of neuroticism can be attributed to genetic factors.
A study with positron emission tomography has found that healthy subjects that score high on the NEO PI-R neuroticism dimension tend to have high altanserin binding in the frontolimbic region of the brain  an indication that these subjects tend to have more of the 5-HT2A receptor in that location.
Another study has found that healthy subjects with a high  neuroticism score tend to have higher DASB binding in the thalamus,  with DASB being a ligand that binds to the serotonin transporter protein.
Another neuroimaging study using magnetic resonance imaging to measure brain volume found that the brain volume was negatively correlated to NEO PI-R neuroticism when correcting for possible effects of intracranial volume, sex, and age.
Other studies have associated neuroticism with genetic variations, e.g.,  with 5-HTTLPR  a polymorphism in the serotonin transporter gene.
However, not all studies find such an association.
A genome-wide association study (GWA study) has pointed single-nucleotide polymorphisms in the MDGA2 gene as associated with neuroticism.
Another GWA study gave some evidence that the rs362584 polymorphism in the SNAP25 gene was associated with neuroticism.

# Geography
Neuroticism, along with other personality traits, has been mapped across states in the USA.
People in eastern states such as New York, New Jersey, West Virginia, and Mississippi tend to score high on neuroticism, whereas people in many western states, such as Utah, Colorado, South Dakota, Oregon, and Arizona score lower on average.
People in states that are higher in neuroticism also tend to have higher rates of heart disease  and lower life expectancy.