Personality neuroscience uses neuroscientific methods to study the neurobiological mechanisms underlying individual differences in stable psychological attributes. Specifically, personality neuroscience aims to investigate the relationships between inter-individual variation in brain structures as well as functions and behavioral measures of persistent psychological traits, broadly defined as "predispositions and average tendencies to be in particular states", including but are not limited to personality traits, sociobehavioral tendencies, and psychopathological risk factors. Personality neuroscience is considered as an interdisciplinary field integrating research questions and methodologies from social psychology, personality psychology, and neuroscience. It is closely related to other interdisciplinary fields, such as social, cognitive, and affective neuroscience.
History
Personality neuroscience is a field built upon the study of personality, which has been a central theme in psychology and evolving through various theoretical perspectives as well as methodological approaches over many years. Specifically, personality neuroscience aims to understand what neurobiological mechanisms underlie and contribute to personality, and therefore, is primarily based on theories that attribute individual differences to physiological and biological systems of the human body or brain. These theories can be traced back to many theories proposed by early physicians, philosophers, and psychologists. Ancient Greek physician Hippocrates developed the theories of Humorism by identifying four vital bodily "humors" or fluids (i.e., blood, phlegm, black bile, and yellow bile) to be associated with temperaments (i.e., sanguine, phlegmatic, melancholic, and choleric, respectively) as well as physical health outcomes. In the early 20th century, the psychoanalytic theories put forth by Austrian neurologist Sigmund Freud was anchored on the unconscious mental processes. Influenced by the psychoanalytic theories, American psychologist Henry A. Murray proposed five principles of personology, his term for the study and system of personality, in which the first principle states that "personality is rooted in the brain. The individual's cerebral physiology guides and govern every aspect of personality". Relatedly, Murray also suggested that "needs", which is the motivation that drive behaviors, arose as a result of "a physiochemical force in the brain". American psychologist William Sheldon was known for his work on defining three "somatotypes" (i.e., body types: endomorphs, mesomorphs, and ectomorphs) to personality attributes.
As early as the late 19th century, the case study of Phineas Gage, a railroad worker who survived a severe brain injury from an accident and underwent a significant personality change, was the first to suggest a causal link between the brain and personality. In the 1940s, there were studies investigating the association between brain wave patterns and individual differences using twin study paradigms, demonstrating that identical twins showed remarkably similar brain wave patterns measured by electroencephalography (EEG) when compared to fraternal twins. However, results from these studies were deemed hard to interpret "in the absence of any satisfactory theory linking brain-wave patterns to personality". Building off these studies and other studies that investigated the genetic inheritance of psychological attributes, in 1951, Hans J. Eysenck and D.B. Prell experimentally tested the heredity of neuroticism using a twin study paradigm and concluded that "the factor of neuroticism is not a statistical [artifact], but constitutes a biological unit which is inherited as a whole" and "the neurotic predisposition is to a large extent hereditarily determined". Following this work, Eysenck continued to investigate psychological traits in relation to neurobiological systems, including the nervous systems, arousal, and brain structures (e.g., reticular formation and the limbic system). In 1961, American psychologist Gordon W. Allport defined personality as "the dynamic organization within the individual of those psychophysical systems that determine his characteristic behavior and thought", localizing personality within "psychophysical systems".
Development Extending from Eysenck's theory on the biological basis of personality, Jeffrey A. Gray's reinforcement sensitivity theory of personality and his work that studied the neural mechanisms underlying personality traits set the foundation for the contemporary field in personality neuroscience. For example, Gray's work suggested that introversion involved both the ascending reticular activating system and an inhibitory system of brain areas including the orbital frontal cortex, medial septal area, and. the hippocampus. In 1999, a chapter titled "The neuroscience of personality" written by Alan D. Pickering and Jeffrey A. Gray was published in the Handbook of personality: Theory and research, in which it introduced ways to "build a modern, integrated neuroscience of personality".
Although there had long been theoretically driven interests and experimental endeavors to understand the neurobiological basis of personality, it wasn't until recent years that, with the advancement in neuroscientific methodologies (e.g., non-invasive neuroimaging methods), the focus of personality psychology began to shift from observing, describing, and categorizing the phenomenon of individual differences towards discovering what may contribute to these observed individual differences. In 2010, the name "personality neuroscience" was coined by Colin G. DeYoung, who is a psychology professor and the director of the DeYoung Personality Lab at the University of Minnesota. In 2018, the Personality Neuroscience journal was established to "[publish] papers in the neuroscience of personality (including cognitive abilities, emotionality, and other individual differences) concerned with understanding causal bases" with "its focus on the equal importance of personality and neuroscience".
Research methodologies As personality neuroscience seeks to understand the link between personality and its underlying neurobiological mechanisms, generating testable hypotheses involve both the measurements of personality attributes and neurobiological structures and/or functions.
Measuring personality In the field of personality psychology, there have been two main approaches to define personality traits:
Nomothetic approach defines personality traits in terms of dimensions and factors, or typologies comprising an organized group of trait-like characteristics, that are generalizable and universal to all people. Along with the development of the factor analysis and psychometrics, the nomothetic approach has been dominant in personality psychology as it offers quantitative measures of traits that can be easily incorporated in research designs and statistical analyses. Two popular models of personality traits are Eysenck's three-factor model (including three factors: neuroticism, extraversion, and psychoticism) and Goldberg's Big Five or McCrae and Costa's five-factor model (including five factors: extraversion, agreeableness, conscientiousness, neuroticism, and impulsivity/openness), in which the latter two are made up of slightly different dimensions but often conceptualized interchangeably in the literature. Just as the Eysenck model and Golderberg/McCrae and Costa's model disagree fundamentally on the numbers of factors (three versus five, respectively), there exists ongoing debates about the numbers of orthogonal factors that may be sufficient to define the personality space. In recent years, a hierarchal model of the Big Five personality is proposed, grouping the five factors (or "domains") into two higher-order "metatraits": stability (i.e., agreeableness, conscientiousness, and reverse-coded neuroticism) and plasticity (i.e., extraversion and openness/intellect) while dividing each domain into two aspects associated with different facets. Idiographic approach emphasizes individuality and defines traits, including individualized traits and the pattern or organization of a combination of traits, in relation to a particular person. Allport, endorsing the idiographic approach, wrote that "the outstanding characteristic of man is his individuality", and his trait theory—centering on the cardinal, central, and secondary traits—were defined in idiographic terms. Personality research that adopted the idiographic approach have demonstrated that the nomothetic approach (e.g., the Big Five) may not be able to capture the within-person personality structure.
In personality neuroscience, personality is often defined using the nomothetic approach. Personality trait is typically measured using scales developed for the personality attributes of interests and administered through self-report surveys and questionnaires. One of the most commonly used ways to measure personality attributes in personality neuroscience research is the Big-Five personality traits. In addition to the criticism by proponents of the idiographic approach as mentioned above, self-report measures on personality traits in general are susceptible to response biases (e.g., social desirability bias, acquiescent response bias, etc.) and inaccurate introspection of mental states. Therefore, it is important to establish construct validity of the self-report measures of personality by using other scales of the same construct or other modalities of measures, such as behavioral data or aggregated ratings from other knowledgeable informants.
Another common nomothetic approach is the Affective Neuroscience Personality Scales (ANPS). The ANPS was originally published in 2003 and was used by neuroscientists to evaluate the primary emotional systems that underlie mental well-being and affective brain disorders. This scale was created by Jaak Panksepp so that researchers could use this self-report test to measure differences in the primary emotions, SEEKING, LUST, CARE, PLAY (the positive emotions) and FEAR, SADNESS, ANGER (the negative emotions). These differences in the emotions were then compared to the Big Five personality to look at the scale from an evolutionary perspective as the primary emotions were seen as a survival mechanism of inherited behavioral patterns by humans interacting with their environment. Each of these primary emotions have "been evolutionarily shaped in terms of inherited tools for survival and, more generally, for fitness" and are seen to regulate human nature. The SEEKING energy is used to seek valuable resources for survival, such as food, a mate, or shelter. The LUST energy is used to sustain the human species through reproductive means. The CARE system is significant in order to protect offspring so that they can grow into adults and the species is sustained once again. PLAY is important in order to foster social bonding between humans, to learn social and motor skills, and to regulate emotions. On the other hand with negative emotions, the FEAR energy is used for safety and to keep away from danger through means such as the flight or fight response. The SADNESS system, from an evolutionary perspective, is used to maintain socialness of an individual, as being isolated often evokes this emotion. The ANGER energy is important to protect resources from others or the environment.