New York Longitudinal Study (NYLS)
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It can be defined as Study of Temperament during infancy, childhood, adolescence and young adulthood.
The New York Longitudinal Study (NYLS), launched by Alexander Thomas and Stella Chess in 1956, marks the beginning of modern interest in the study of temperament. Although the scientific study of temperament is relatively recent, the idea of grouping human beings into basic behavioral types is centuries old. Historically, temperament refers to those biologically based differences between individuals that emerge early in life and are expressed with relative consistency across situations and over time. Galen described four basic temperaments – choleric, melancholic, sanguine, and phlegmatic – attributable to a preponderance of one or another of Hippocrates’s four cardinal humors, black bile, yellow bile, phlegm, and blood. In the early years of the twentieth century, Kretchmer in Germany and Sheldon in the United States examined the relationship between basic temperaments and endomorphic, mesomorphic, and ectomorphic body types. As the century progressed, individual differences with respect to a range of discrete functional areas including motility, perceptual responses, sleeping and feeding patterns, autonomic response patterns, and biochemical individuality had begun to be described in infants and young children. However efforts to relate early individual differences to later psychological organizations were only minimally successful (Thomas et al. 1960).
Thomas and Chess did not set out to study temperament. Dissatisfied with the prevailing wisdom of the time which held that individual differences in behavior resulted from the differential impact of environment and experience during development, they sought to explore what children might contribute to their own development. By identifying aspects of behavioral individuality early in life and tracing their vicissitudes over time, these investigators sought to (1) determine the persistence of these initially identified characteristics and (2) delineate their pertinence to later psychological organizations including the possible emergence of psychopathology. Thomas and Chess were clinicians, not trained investigators. Nevertheless they were committed to the development of a research methodology that would insure that the techniques used to gather and analyze data were reliable, valid, and reproducible by others. Their success in this regard accounts for the success of the investigation and the ongoing influence of its findings (Hertzig 2012).
Subjects and Methods
The NYLS sample consisted of 133 children (67 males, 66 females) from 85 families, recruited over 12 years. When enrolled, subjects, who derived from intact, highly educated, and economically well-situated families, were under 3 months of age. Parents were the primary source of information about their children. A semi-structured interview protocol, informed by the Gesell Behavior Day (1942), was developed, and parents were interviewed when their children were 3 months of age and at 3-month intervals during the first year and at 6-month intervals until 5 years and yearly thereafter. Parents were guided to provide details of their child’s behavior during activities of daily living including sleep, feeding, dressing and undressing, bathing, responses to sensory stimuli, new situations, and illnesses. As the children grew older, parents were questioned about additional developmentally appropriate activities including play patterns, responses to discipline, acquisition of self-care skills, and peer relationships. Individually administered cognitive assessments were obtained at 3, 6, and 9 years of age, and the children were observed and their teachers interviewed in nursery school, kindergarten, and 1st grade. Psychiatric consultation was made available “on demand.” At 16 years, children and parents were interviewed separately about school functioning, extracurricular activities, sexual behavior, drug use, current problems, and future plans. A specially designed measure of adult temperament was administered when subjects were between 18 and 22 years of age at which time they were again interviewed individually. A narrative summary of all contacts with informants was prepared within 24 h (Thomas et al. 1963; Thomas and Chess 1977, 1984).
Analysis of Data
Activity level: The motor component present in a child’s functioning.
Rhythmicity (regularity): The predictability or unpredictability in time of any biologic function.
Approach or withdrawal: The nature of the initial response to a new stimulus (e.g., new food, new toy, new person). Approach responses are positive and withdrawal reactions negative, whether displayed by mood expression or motor activity.
Adaptability: This category describes the ease with which an initial response (irrespective of its character) can be modified in the desired direction.
Threshold of responsiveness: The intensity level of stimulation that is necessary to evoke a discernible response regardless of the specific form that the response may take or the sensory modality affected.
Intensity of reaction: The energy level of response regardless of its quality or direction.
Quality of mood: The amount of pleasant, joyful, and friendly behavior, as contrasted with unpleasant, crying, or unfriendly behavior.
Distractibility: The effectiveness of extraneous environmental stimuli in interfering with or altering the direction of the ongoing behavior.
Attention span and persistence: Two categories that are related. Attention span concerns the length of time the child pursues a particular activity; persistence refers to the continuation of an activity direction in the face of obstacles.
Each category was scored on a 3-point Likert scale (high, intermediate, or low), and each category was scored independently to avoid halo effects. Reliability (0.01 level of confidence) was obtained between two independent scorers. Validity of NYLS parent’s reports was assessed through comparison with two direct observations conducted different times within 2 weeks of the parent interviews. Each direct observation was found to agree with the parent interview at the 0.01 level of confidence (Thomas et al. 1963).
Initially, quantitative analysis of the NYLS data was directed toward the exploration of stability of temperamental attributes over time. Year-to-year consistency of each of the nine temperamental attributes was examined through the first 5 years of life using correlational techniques. Correlations from 1 year to the next ranged from 0.05 to 0.51. All were statistically significant with the exception of approach/withdrawal (years 1–2, 2–3), distractibility (years 1–2, 2–3, 3–4), and persistence (years 1–2, 4–5). As the time between years compared was increased to 2, 3, or 4 years, the number of significant correlations decreased accordingly (Thomas and Chess 1977, p. 161, Thomas and Chess 1982).
Factor analysis of the nine individual temperamental attributes led to the identification of three temperamental constellations: Children with an easy temperament (40%) were characterized by regularity, positive approach responses to new stimuli, rapid adaptability to change, and mild or moderate predominantly positive intensity of mood. Children described as temperamentally difficult (10%) displayed irregular biological functions, predominantly negative withdrawal responses to new situations, slow adaptability to change, and intense expression of predominately negative mood. Children who were slow to warm up (15%) displayed the temperamental attributes of initial withdrawal to new situations, combined with slow adaptability and mild intensity. The remaining 35% displayed different combinations of temperamental attributes, so in order to include the entire sample in subsequent quantitative analyses, an index of difficulty was constructed for each child by determining the means of the scores of the five categories that made up the difficulty child temperamental constellation. The pattern of inter-year correlations for the index of difficulty was similar to that obtained when each temperamental attribute was considered separately. Although few significant correlations between individual temperamental attributes during childhood and early adulthood were obtained, difficult temperament at year 3 was found to be significantly negatively correlated with an omnibus measure of early adult adjustment (Chess and Thomas 1990, p. 207).
These results suggest that consistency of temperamental attributes over time is unlikely to account for more that 10–15% of the variance. Thomas and Chess have proposed that genetic, developmental, and environmental factors all contribute to this substantial potential for change. Additionally they have emphasized that the temperamental differences between NYLS subjects, whether described qualitatively as easy, difficult, or slow to warm up or quantitatively by the magnitude of the index of difficulty, are to be understood as variations within the range of usually expectable behavior. Even a relatively extreme score for a specific temperamental attribute is not, in and of itself, to be considered as evidence for psychopathology (Chess and Thomas 1990).
Nevertheless, the risk of developing a clinically significant behavior disorder was different for children with different temperamental characteristics. The difficult children were at the greatest risk; of the 10% of the NYLS sample who were characterized as difficult, 71% were found to have behavior problems. In the much larger group of easy children (40% of the total NYLS sample), behavior problems were found in less than 7%. The slow-to-warm-up children were also at somewhat higher risk for behavior disorder, with approximately 50% becoming clinical cases. To account for this pattern of differential risk, the NYLS investigators advanced the concept of “goodness of fit” which occurs when the attributes and capacities of the individual are in accord with the demands and expectations of the environment. Such consonance between the characteristics of the individual and environmental expectations potentiates optimal positive development, whereas dissonance between individual attributes and environmental expectations increases the likelihood of maladaptive functioning and distorted development (Chess and Thomas 1984; Thomas et al. 1968).
It has been somewhat more than half a century since Alexander Thomas and Stella Chess first sought to examine what it was that children might bring to their own development. In the years since the initiation of the NYLS temperament, researchers have been focused on how to define temperament and to specify and measure the developmental course of its dimensions. Clinically, Chess and Thomas have called attention to relations between temperamental attributes – most particularly those of the difficult child – and the emergence of behavioral disturbance. The concept of “goodness of fit” has also been used to provide an organizing framework for parental guidance, as a treatment modality for childhood behavior disorders and as part of preventative efforts in such high-risk populations as low birth weight infants and babies of teen-aged mothers. The focus of investigations of relations between temperament and psychopathology is currently expanding to include a focus on the neural circuitry underlying different temperamental dimensions to further illuminate their contributions to the etiology, pathogenesis, and treatment of behavioral and emotional disorders as they present in children and adolescents (Hertzig 2012).
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