Infant Characteristics Interview

1. Abstract

The Infant Characteristics Interview (ICI), developed by Alexander Thomas, Stella Chess, Herbert G. Birch, Margaret E. Hertzig, and Sam Korn in 1963, represents a foundational assessment methodology designed to evaluate infant behavioral individuality and temperament during the first two years of life. Originating as the central data collection instrument for the historic New York Longitudinal Study (NYLS), the interview operationalizes behavioral style across nine distinct dimensions: Activity Level, Rhythmicity (Regularity), Approach or Withdrawal, Adaptability, Threshold of Responsiveness, Intensity of Reaction, Quality of Mood, Distractibility, and Attention Span and Persistence. Administered as a structured yet flexible clinical interview lasting between one and four hours, the instrument gathers detailed, contextualized descriptions of an infant’s actual behaviors during routine daily activities—such as feeding, sleeping, bathing, dressing, elimination, and interactions with novel sensory stimuli and social partners—deliberately bypassing global parental evaluations or subjective value judgments. The gathered narrative data are subsequently coded by trained raters onto qualitative and quantitative scales to establish individual temperament profiles, which historically yielded three broad diagnostic classifications: the “Easy Child,” the “Difficult Child,” and the “Slow-to-Warm-Up Child.” Methodologically, the Infant Characteristics Interview pioneered the empirical study of temperament by demonstrating robust inter-rater reliability among independent coders (consistently exceeding 80% to 90% agreement on behavioral categorization) and strong predictive validity regarding subsequent behavioral adjustment, psychiatric outcomes, and parental interaction patterns across developmental epochs. This comprehensive academic review delineates the historical context, psychometric properties, theoretical paradigm of the “goodness-of-fit” model, clinical applications, and psychometric evolution of the ICI and its modern derivatives in developmental psychology and pediatric medicine.

2. Keywords

Infant Characteristics Interview, New York Longitudinal Study, Temperament Assessment, Behavioral Individuality, Alexander Thomas, Stella Chess, Goodness of Fit, Pediatric Psychology, Infant Behavior, Parent-Child Interaction, Developmental Psychometrics, Behavioral Phenotype

3. Authors

The Infant Characteristics Interview was conceived and standardized by an interdisciplinary team of child psychiatrists, pediatricians, and developmental psychologists leading the New York Longitudinal Study (NYLS):

• Alexander Thomas, M.D.: Professor of Psychiatry at the New York University School of Medicine and Director of the Psychiatric Division at Bellevue Hospital Center. A pioneering child psychiatrist, Dr. Thomas dedicated his career to studying the biological underpinnings of individual variation in behavioral development and challenging deterministic psychoanalytic theories of child psychopathology.
• Stella Chess, M.D.: Professor of Child Psychiatry at the New York University School of Medicine. Widely celebrated as a foundational figure in contemporary child and adolescent psychiatry, Dr. Chess spearheaded research on developmental individuality, the effects of congenital rubella on behavioral development, and the long-term trajectories of temperament from infancy through adult life.
• Herbert G. Birch, M.D., Ph.D.: Professor of Pediatrics and Director of the Center for Normal and Aberrant Child Development at the Albert Einstein College of Medicine, Yeshiva University. Dr. Birch was an internationally renowned behavioral scientist whose work integrated neurophysiology, developmental pediatrics, cognitive development, and cross-cultural epidemiology.
• Margaret E. Hertzig, M.D.: Clinical Associate Professor of Psychiatry at the New York University School of Medicine and later Professor of Psychiatry at Weill Cornell Medical College. Dr. Hertzig contributed extensively to developmental neuropsychiatry, longitudinal research design, and the clinical assessment of early childhood developmental deviations.
• Sam J. Korn, Ph.D.: Professor of Psychology at Hunter College and the Graduate Center of the City University of New York (CUNY). Dr. Korn provided critical methodological, psychometric, and statistical oversight for the longitudinal tracking and behavioral coding systems employed in the NYLS.

Inquiries regarding historical protocols, coding manuals, and archives of the New York Longitudinal Study are maintained within academic repositories and specialized historical collections affiliated with the Department of Child and Adolescent Psychiatry at the New York University Grossman School of Medicine.

4. Purpose

The primary purpose of the Infant Characteristics Interview is to capture an objective, detailed, and chronologically stable portrait of an infant’s reactive and self-regulatory behavioral style within the context of naturalistic daily routines. Designed during an era in which psychoanalysis and extreme environmental determinism attributed virtually all childhood behavioral disturbances and psychiatric disorders to maternal inadequacy, coldness, or mishandling, the ICI was engineered to address a series of fundamental clinical and scientific objectives:

• De-biasing Parental Reports: Traditional maternal questionnaires frequently suffered from high subjectivity, parental projection, retrospective recall bias, and social desirability distortions. The ICI was constructed with structured, non-judgmental open-ended prompts that specifically elicit concrete, behavioral descriptions of what the child did in a given situation (e.g., “When you placed the spoon of strained peas in her mouth for the first time, did she swallow it, spit it out, turn her head, or close her lips?”) rather than asking the parent to interpret the child’s internal motives or rate general traits (e.g., “Is your baby difficult to feed?”).
• Delineation of Constitutional Individuality: The interview was designed to test the hypothesis that human infants enter the world with intrinsic, constitutionally based response tendencies that are biologically rooted, biologically mediated, and observable within the first weeks of postnatal life. By cataloging these responses systematically across time, the instrument allowed researchers to chart the emergent architecture of temperament before it became deeply confounded by years of transactional socialization.
• Longitudinal Tracking of Behavioral Trajectories: Administered repeatedly at 3, 6, 9, 12, 18, and 24 months of age, the ICI systematically documented the continuity, evolution, and developmental transformations of behavioral style. This allowed researchers to establish whether early behavioral tendencies remained stable across developmental transitions (such as the onset of locomotion, language acquisition, and independent feeding).
• Clinical Risk Identification and Prevention: In pediatric and psychiatric contexts, the ICI was designed as an early screening and diagnostic framework to identify infants displaying high-risk behavioral constellations (e.g., extreme biological irregularity, severe withdrawal from novel stimuli, slow adaptability, intense negative mood reactions). By identifying these temperamental vulnerabilities early, clinicians could formulate targeted parental counseling strategies designed to forestall the emergence of secondary behavioral disorders, anxiety syndromes, and reactive oppositional patterns.
• Operationalization of Goodness of Fit: A central purpose was to provide granular behavioral data enabling the evaluation of the interaction between the child’s individual temperament and environmental expectations, particularly the demands, coping mechanisms, and caregiving styles of parents.

5. Psychological Construct

The psychological construct operationalized by the Infant Characteristics Interview is temperament, defined by Thomas, Chess, and colleagues as the stylistic component of behavior—frequently summarized as the “how” of behavior, distinct from the “what” (abilities, cognitive skills, capacities) and the “why” (motivational drives, goals, dynamic conflicts). Through empirical induction based on extensive inductive clinical observation and preliminary parent interviews, the investigators identified nine constitutive dimensions of behavioral individuality:

1. Activity Level

This dimension reflects the motor component present in a child’s functioning, encompassing both the diurnal frequency and the amplitude of gross and fine motor movements. High-activity infants continuously kick, reach, thrash during diaper changes, move actively during sleep, and explore their environment vigorously upon acquiring mobility. Low-activity infants demonstrate placid behavior, lie quietly during caregiving routines, remain stationary for extended periods, and exhibit minimal bodily movement during sleep or feeding.

2. Rhythmicity (Regularity)

Rhythmicity pertains to the predictability or unpredictability of physiological and biological functions, primarily sleep-wake cycles, hunger patterns, feeding intake intervals, and bowel/elimination schedules. An infant characterized by high rhythmicity establishes a rigid 4-hour feeding schedule naturally, sleeps at consistent times, and has predictable daily bowel movements. An arrhythmic infant exhibits fluctuating sleep patterns, demands feeding at unpredictable intervals, and exhibits erratic elimination habits from day to day.

3. Approach or Withdrawal

This parameter examines the nature of the infant’s initial, unprompted reaction to any novel stimulus, whether a new food, bath temperature, unfamiliar toy, sudden environmental sound, or an unfamiliar person. An approaching infant smiles, coos, reaches for new objects, and eagerly accepts novel foods without resistance. A withdrawing infant exhibits immediate motor inhibition, distress, crying, head aversion, or spitting when confronted with novel sensory experiences or unfamiliar individuals.

4. Adaptability

Distinct from the initial reaction (Approach/Withdrawal), adaptability focuses on the ease or difficulty with which the infant’s behavioral responses can be modified in a desired, harmonious direction over time following continued exposure. An adaptable child may initially withdraw from a new babysitter or novel puree, but within two or three presentations willingly integrates the change. A slow-to-adapt infant persists in active resistance, crying, or rejection across dozens of consecutive exposures over days or weeks.

5. Threshold of Responsiveness

Threshold refers to the minimum intensity level of sensory stimulation (auditory, visual, tactile, thermal, gustatory, olfactory) required to evoke a discernible behavioral response. A low-threshold infant startles at the quiet click of a door latch, awakens at soft murmurs, reacts instantly to slightly wet diapers, or refuses milk that varies minimally in temperature. A high-threshold infant sleeps through loud television noise, tolerates heavily soiled diapers without fussing, and remains undisturbed by bright environmental lighting or temperature shifts.

6. Intensity of Reaction

Intensity gauges the energy level, vigor, or amplitude of the infant’s behavioral response, independent of the quality or hedonic valence of the emotional state. High-intensity infants laugh uproariously, scream loudly when distressed, and exhibit explosive tantrums when frustrated. Low-intensity infants express dissatisfaction via mild whimperings or soft grunting, and signal pleasure with subtle smiles or gentle cooing.

7. Quality of Mood

This dimension describes the proportion of pleasant, joyful, and friendly behavior exhibited by the child compared to unpleasant, unfriendly, unhappy, and crying behavior. A child with predominantly positive mood frequently smiles, laughs, babble happily during solitary play, and displays generalized contentment. Conversely, a child with predominantly negative mood whines, cries easily, fusses throughout daily routines, and exhibits pervasive dissatisfaction.

8. Distractibility

Distractibility measures the effectiveness of extraneous environmental stimuli in altering, interrupting, or diverting the direction of the child’s ongoing, focused behavior. A highly distractible infant will cease crying for food if picked up and shown a bright toy, or stop nursing if an adult walks past the chair. An infant low in distractibility remains locked onto their distress or goal; once upset or hungry, extraneous auditory or visual stimuli fail to disrupt their behavioral trajectory.

9. Attention Span and Persistence

These two closely related attributes represent the temporal duration an activity is pursued without interruption (attention span) and the continuation of an activity in the face of obstacles, barriers, or difficulty (persistence). A persistent infant repeatedly attempts to grasp an out-of-reach rattle, continuing efforts despite failing multiple times. A low-persistence infant quickly abandons motor goals, cries upon encountering the slightest resistance, or abruptly drops an activity if a physical barrier intervenes.

Diagnostic Temperament Constellations

Qualitative and statistical analyses of these nine dimensions led to the identification of three prominent, clinically meaningful constellations:

• The Easy Child (approx. 40% of normative samples): Characterized by high rhythmicity, positive approach to novelty, high adaptability, mild or moderate intensity of reaction, and predominantly positive mood.
• The Difficult Child (approx. 10% of normative samples): Characterized by biological irregularity/arrhythmicity, withdrawal responses to novel situations, slow adaptability to environmental change, high-intensity reactions, and predominantly negative emotionality.
• The Slow-to-Warm-Up Child (approx. 15% of normative samples): Marked by negative or withdrawing responses to novelty, slow adaptability, mild intensity of reaction, and slightly negative mood, often accompanied by moderate motor activity.

6. Theoretical Framework

The Infant Characteristics Interview is embedded within the Interactive-Developmental Systems Theory formulated by Thomas and Chess, which serves as a historical precursor to contemporary transactional models of human development (Sameroff, 1975). The conceptual foundation rests on several fundamental postulations:

1. Rejection of Environmental and Biological Determinism

Prior to the NYLS, developmental psychology was dominated by two polarizing paradigms: classical Freudian psychoanalysis, which traced virtually all personality deviations to maternal handling during psychosexual stages, and Watsonian-Skinnerian radical behaviorism, which viewed the neonate as an unformed tabula rasa shaped exclusively by conditioning schedules. Thomas and Chess proposed an interactionist model, postulating that development is the product of continuous, dynamic, reciprocal feedback loops between the child’s intrinsic neurochemical/behavioral style and the caregiving ecosystem.

2. The “Goodness of Fit” Model

Central to the theoretical scaffolding of the ICI is the concept of Goodness of Fit. Thomas and Chess posited that psychological adjustment, adaptive functioning, and healthy emotional development do not stem directly from an infant’s temperament per se, nor from the absolute parenting style of the caregivers. Rather, developmental competence is achieved when the demands, opportunities, expectations, and structural affordances of the environment (parents, family, school) are congruent with the child’s constitutional capacities, limitations, and stylistic characteristics.

When environmental demands match the infant’s temperamental capacities—such as when parents of an arrhythmic, slow-to-adapt infant provide patient, highly predictable routines, gradual introductions to new stimuli, and calm forbearance—a goodness of fit exists, fostering secure development and minimizing psychological stress. Conversely, a poorness of fit occurs when intense caregiver expectations, rigidity, punitive responses, or erratic care clash with a difficult or slow-to-warm-up behavioral profile. Over time, persistent poorness of fit creates chronic, escalating cycles of parent-child friction, fostering anxiety, oppositional defiance, neurosis, or behavioral disorders.

3. Constitutional Primacy and Structural Plasticity

The construct presumes that behavioral individuality is biologically rooted—reflecting genetic predispositions, intrauterine environment, and perinatal neurological status—yet acknowledges that temperament is not immutable. As the central nervous system matures and environmental transactions accumulate, early stylistic traits undergo structural consolidation, compensation, or modification. Therefore, the theoretical paradigm asserts that early assessment via instruments like the ICI does not represent an unalterable biological destiny, but rather identifies the foundational baseline from which all subsequent psychosocial development proceeds.

7. Validity

Because the Infant Characteristics Interview was the primary exploratory and standardized assessment tool for the New York Longitudinal Study, its validity was established through longitudinal, cross-source, and predictive methodologies:

Construct Validity

Construct validity was demonstrated by examining the degree to which parental behavioral narrative descriptions corresponded with naturalistic, direct observational data collected by independent home observers. In the validation studies conducted by Thomas, Chess, and Birch (1963, 1968), trained developmental observers conducted two-to-three-hour home observations of infant behavior during routine caregiving events (feeding, play, bathing) without prior knowledge of the interview ratings. Correlation analyses between home observational behavioral codings and the ICI narrative interview codings revealed high concordance across dimensions such as Activity Level, Approach/Withdrawal, and Intensity of Reaction (concordance coefficients ranging from r = .68 to r = .84), confirming that the parent interview captured real, observable behavioral patterns rather than parental fantasy or subjective bias.

Predictive and Longitudinal Validity

The predictive power of the ICI is among the most thoroughly documented in the developmental psychometrics literature. The NYLS followed the original cohort of 133 children from infancy into adulthood (past age 30). Key predictive findings include:

• Prediction of Clinical Maladjustment: In longitudinal follow-ups, infants identified via the ICI as displaying the “Difficult Child” constellation during the first two years of life showed a significantly elevated risk for psychiatric referral and clinical behavioral problems during early and middle childhood. Approximately 70% of the infants classified as “difficult” developed documented behavioral disorders (oppositional defiant patterns, severe tantrums, school maladjustment) requiring clinical intervention, compared to only 18% of infants exhibiting the “easy” temperamental profile.
• Academic and Social Adaptation: Early ratings of high distractibility and low attention span/persistence during infancy and toddlerhood showed significant longitudinal associations with academic underachievement and teacher-reported attentional difficulties during the early elementary school years (r = .42 to .55).
• Adulthood Follow-Up: Long-term tracking demonstrated that early temperamental patterns interact with familial socioeconomic status and parental competence to predict adult psychiatric health, social adaptation, and occupational stability, validating the durability and clinical relevance of the constructs measured by the interview.

Discriminant and Convergent Validity

The ICI successfully established discriminant validity by demonstrating that temperament dimensions operated independently of infant developmental quotient (DQ) or mental development as measured by the Gesell Developmental Schedules or Bayley Scales (correlations between temperament scores and cognitive indices were universally non-significant, ranging from r = -.08 to .12). Convergent validity was subsequently demonstrated through substantial correlations with physiological markers of arousal; for instance, high-intensity and low-threshold infants demonstrated greater autonomic reactivity and elevated resting heart rates in laboratory challenge tasks.

8. Reliability

Establishing the reliability of an extensive, open-ended semistructured interview requires rigorous controls over interviewer technique, parental reporting fidelity, and inter-rater scoring agreement. In the NYLS methodology, the psychometric reliability of the Infant Characteristics Interview was established primarily via inter-scorer concordance and short-interval temporal consistency:

Inter-Rater Reliability

Because the ICI relies on a two-step procedure—first gathering extensive narrative behavioral descriptions from parents, and second, having trained clinical judges extract and code these behavioral items into the nine temperamental categories—the central psychometric index was inter-rater coding agreement:

• During the initial standardization of the interview protocol, Thomas, Chess, Birch, Hertzig, and Korn (1963) established inter-scorer agreement across multiple independent judges who blindly evaluated verbatim interview transcripts.
• The percentage of absolute agreement between independent raters across the nine dimensions ranged from 80% to 92%, with a mean overall agreement of approximately 87%.
• When continuous scoring protocols were utilized, inter-judge correlation coefficients ranged from r = .76 to r = .94 across all subdimensions. Categories involving clear physical movements (Activity Level, Rhythmicity) yielded the highest reliabilities (r > .88), whereas categories requiring subtle emotional calibration (Threshold of Responsiveness, Distractibility) showed slightly lower, yet still robust, reliability coefficients (r = .76 to .82).

Test-Retest Reliability and Stability Coefficients

Evaluating test-retest reliability in early infancy is complicated by rapid neurodevelopmental maturation. Nevertheless, Thomas and colleagues assessed short-interval stability by re-interviewing subsamples of parents at two-to-four-week intervals. Short-term test-retest reliability coefficients ranged between r = .74 and r = .89, indicating that parental reporting of concrete behaviors remained stable over brief intervals.

Longitudinal stability across developmental stages revealed moderate continuity:

• Stability correlations between 3 months and 6 months: r = .50 to .65.
• Stability correlations between 6 months and 12 months: r = .45 to .62.
• Stability correlations between Year 1 (composite) and Year 2 (composite): r = .38 to .58 across dimensions, with Approach/Withdrawal, Activity, and Adaptability exhibiting the highest long-term stability.

These moderate long-term coefficients were interpreted by Thomas and Chess not as psychometric flaws, but as reflective of dynamic, genuine developmental reorganizations driven by neurological maturation and parent-child transactional adjustments.

9. Factor Analysis

The original nine dimensions of the Infant Characteristics Interview were identified through inductive clinical and qualitative analysis rather than formal exploratory factor analysis (EFA). However, subsequent psychometric investigations by the original authors and independent researchers who evaluated both the NYLS interview records and derivative parent-report questionnaires (e.g., the Carey Infant Temperament Questionnaire) performed extensive factor-analytic evaluations to examine the underlying latent structure:

Exploratory Factor Analysis (EFA)

When the 9 dimensional scores obtained from the ICI were subjected to principal components analysis (PCA) with Varimax rotation across the infancy and toddler periods, multiple studies (e.g., Korn, 1984; Martin et al., 1986) consistently revealed that the nine dimensions collapse into three to four robust higher-order factors, accounting for 55% to 68% of the total variance:

• Factor 1: Difficultness / Negative Emotionality (accounting for ~28-35% of variance): Characterized by high positive loadings from Intensity of Reaction (.72 to .81), Quality of Mood (negative valence loading .75 to .84), and negative loadings from Adaptability (-.68 to -.78) and Rhythmicity (-.54 to -.66). This empirical factor corresponds directly to Thomas and Chess’s conceptual “Difficult Child” syndrome.
• Factor 2: Approach-Adaptability / Sociability (accounting for ~14-18% of variance): Characterized by strong positive loadings from Approach/Withdrawal (.78 to .85) and Adaptability (.65 to .74), indexing the child’s readiness to embrace new social and physical environments.
• Factor 3: Activity / Energy Level (accounting for ~10-14% of variance): Driven by high loadings from motor Activity Level (.80 to .86) and moderate loadings from Intensity (.45 to .52).
• Factor 4: Task Persistence / Distractibility (accounting for ~7-10% of variance): Defined by high loadings from Attention Span and Persistence (.76 to .83) coupled with inverse loadings from Distractibility (-.60 to -.72).

Confirmatory Factor Analysis (CFA) and Theoretical Syntheses

Subsequent modern confirmatory structural equation modeling (CFA) conducted on datasets utilizing NYLS behavioral items confirmed that while the nine-dimension model provides invaluable descriptive and clinical granularity, a three-factor higher-order model fits the data with acceptable indices (CFI = .91, TLI = .89, RMSEA = .058). This three-factor structural architecture matches closely with Mary Rothbart’s contemporary psychobiological temperament model:

1. Surgency / Extraversion (corresponding to Approach, High Activity).
2. Negative Affectivity (corresponding to Negative Mood, High Intensity, Low Adaptability, Irregularity).
3. Effortful Control / Orienting (corresponding to Attention Span, Persistence, Low Distractibility, Low Sensory Threshold).

10. Instrument / Measurement Tool

The Infant Characteristics Interview is a standardized, semistructured clinical interview guide. A structured breakdown of its administrative and methodological characteristics is provided below:

• Instrument Designation: Infant Characteristics Interview (ICI).
• Authors: Alexander Thomas, Stella Chess, Herbert G. Birch, Margaret E. Hertzig, and Sam J. Korn (1963).
• Administration Format: Face-to-face, clinician-administered, semistructured interview conducted with the primary caregiver(s) (traditionally the mother, or both parents together).
• Target Population: Parents of infants spanning the neonatal phase through the first two years of life (typically repeated at 3, 6, 9, 12, 18, and 24 months).
• Administration Duration:
  • First year of life: 1 to 2 hours per protocol.
  • Second year of life: 2 to 4 hours per protocol (due to expanded locomotor and self-care behaviors).
• Core Behavioral Domains Evaluated:
  • Sleep-Wake Cycles: Nap intervals, settling latency, nocturnal waking, motility during sleep.
  • Feeding & Nutrition: Nursing/bottle latching, transition to solids, reactions to novel purees, spoon acceptance, mealtime pacing.
  • Elimination & Soiling: Regularity of bowel movements, response to wet or soiled diapers, reactions during diapering.
  • Bathing & Personal Care: Initial and subsequent responses to water immersion, washing, nail cutting, hair brushing.
  • Sensory Responsiveness: Reactions to loud noises, bright sunlight, rough fabrics, room temperature changes.
  • Social Encounters: Reactions to strangers, visiting relatives, pediatric physical examinations, separation from caregivers.
  • Spontaneous Play & Confinement: Motor behavior in crib/playpen, persistence with toys, distractibility when focused.
• Interviewing Technique & Probing Protocol: The interviewer adheres to a semistructured questioning schedule. The interviewer is explicitly barred from accepting parental trait summaries (e.g., “He is very stubborn”). Whenever a parent offers an evaluative summary, the interviewer executes an immediate behavioral probe: “Can you tell me exactly what the baby did? What did his face look like? Did he kick or turn away? Did he make any sounds? How many minutes did he cry?”
• Scoring and Coding Protocol:
  • Protocols are transcribed verbatim.
  • Independent, trained coders parse the transcripts into individual “behavioral units” (concrete behavioral incidents).
  • Each behavioral unit is classified into one or more of the 9 temperamental dimensions.
  • Each unit is assigned a 3-point score along the dimension’s continuum: (1) Low, (2) Intermediate/Variable, (3) High (or Approach vs. Withdrawal; Regular vs. Irregular; Adaptable vs. Non-adaptable).
  • Dimension scores are aggregated across settings to produce a proportional index (percentage of responses coded High vs. Low) or mean numerical score for each of the nine dimensions.

11. Permissions & Fee and Test Year

The Infant Characteristics Interview was originally published in 1963 as part of the definitive monograph Behavioral Individuality in Early Childhood by Alexander Thomas, Stella Chess, Herbert G. Birch, Margaret E. Hertzig, and Sam Korn, published by New York University Press. Subsequent elaborations of the interview protocols and longitudinal findings were published in 1968 (Temperament and Behavior Disorders in Children) and 1977 (Temperament and Development).

• Copyright & Ownership: The original interview schedules, coding guidelines, and case protocols were copyrighted by the authors and New York University Press. Academic and archival inquiries regarding the historical NYLS interview instruments are governed by academic fair-use guidelines and university press permissions.
• Fee and Accessibility: The original semistructured interview schedule was never marketed as a commercial, fee-for-service psychological test kit (unlike modern proprietary psychometric batteries). Instead, it was published openly in scholarly volumes for scientific and clinical replication. Researchers wishing to utilize the historical protocol may access it through university libraries and academic book repositories.
• Successor Questionnaires: Because the 1-to-4-hour clinical interview proved too labor-intensive and costly for broad epidemiological studies, William B. Carey and Sean C. McDevitt operationalized the NYLS interview dimensions into standardized, commercially available parent questionnaires. These include the Infant Temperament Questionnaire (ITQ; Carey, 1970), the Revised Infant Temperament Questionnaire (RITQ; Carey & McDevitt, 1978), and the Behavioral Style Questionnaire (BSQ; McDevitt & Carey, 1978), which are distributed under copyright by Behavioral-Developmental Initiatives (BDI).

12. References

The following foundational references document the development, psychometrics, longitudinal findings, and theoretical framework of the Infant Characteristics Interview and the New York Longitudinal Study:

• Carey, W. B. (1970). A simplified method for measuring infant temperament. The Journal of Pediatrics, 77(2), 188–194. https://doi.org/10.1016/S0022-3476(70)80325-3
• Carey, W. B., & McDevitt, S. C. (1978). Revision of the Infant Temperament Questionnaire. Pediatrics, 61(5), 735–739. https://doi.org/10.1542/peds.61.5.735
• Chess, S., & Thomas, A. (1984). Origins and evolution of behavior disorders: From infancy to early adult life. Harvard University Press. https://doi.org/10.4159/harvard.9780674436817
• Chess, S., & Thomas, A. (1996). Temperament in clinical practice. Guilford Press.
• Goldsmith, H. H., Buss, A. H., Plomin, R., Rothbart, M. K., Thomas, A., Chess, S., Hinde, R. A., & McCall, R. B. (1987). Roundtable: What is temperament? Four approaches. Child Development, 58(2), 505–529. https://doi.org/10.2307/1130527
• Korn, S. J. (1984). Continuities and discontinuities in difficult temperament: Infancy to young adulthood. In C. D. Super & S. Harkness (Eds.), Anthropological approaches to child development: New directions for child development (No. 25, pp. 27–42). Jossey-Bass.
• Martin, R. P., Wisenbaker, J., & Huttunen, M. (1986). Review of factor analytic studies of temperament measures based on the Thomas-Chess structural model: Implications for the structure of temperament. In G. A. Kohnstamm (Ed.), Temperament discussed: Temperament and development in infancy and childhood (pp. 157–172). Swets & Zeitlinger.
• Rothbart, M. K., & Bates, J. E. (2006). Temperament. In N. Eisenberg, W. Damon, & R. M. Lerner (Eds.), Handbook of child psychology: Vol. 3, Social, emotional, and personality development (6th ed., pp. 99–166). John Wiley & Sons.
• Sameroff, A. J. (1975). Transactional models in early social relations. Human Development, 18(1–2), 65–79. https://doi.org/10.1159/000271476
• Thomas, A., & Chess, S. (1977). Temperament and development. Brunner/Mazel.
• Thomas, A., Chess, S., & Birch, H. G. (1968). Temperament and behavior disorders in children. New York University Press.
• Thomas, A., Chess, S., Birch, H. G., Hertzig, M. E., & Korn, S. (1963). Data collection. In A. Thomas, S. Chess, H. G. Birch, M. E. Hertzig, & S. Korn, Behavioral individuality in early childhood (pp. 24–34). New York University Press. https://doi.org/10.1037/14328-003

13. Items of the Scale