Potential confounding factors

The observed relationship between child maltreatment, household dysfunction and physical development may be distorted by a third factor. A confounding factor is associated with the exposure (child maltreatment or household dysfunction) and also independently affects the response (e.g. height or onset of puberty). Several potential confounding factors were identified from the literature, and the corresponding variables in the 1958 cohort are described below. Results from chi-squared tests and linear regression models which examined potential confounding factors relationship with exposure and response measures are presented in Appendices 2.5-2.10, and discussed here. Factors which were significantly associated with the exposure, as well as height and pubertal measures were included in the adjusted analyses (Chapters 5 and 6).

2.4.1 Demographic characteristics

Demographic characteristics considered as potential confounding factors were gender and ethnicity. At age 33y, cohort members were asked which ethnic group they

considered themselves belonging to; white, black Caribbean, black African, black other, Indian, Pakistani, Bangladeshi or Chinese. Ethnic group was also self-reported at age 42y. Categories included British, Irish, white other, white and black Caribbean, white and black African, white and Asian, other mixed race, Indian, Pakistani, Bangladeshi, other Asian, Caribbean, African, other black, Chinese or other ethnic group. Two broad categories (white and other) were derived due to small numbers of some ethnic groups. Where responses were inconsistent (n=63), the most recently reported ethnic group (age 42y) was adopted.

2.4.2 Prenatal factors

Maternal height (inches) was measured, without shoes, by the midwife in 1958. Where data were missing, it was supplemented with reported maternal height (inches) in 1969. Paternal height (inches) was reported (predominantly by the mother) in 1969. All height measures were converted to cm and standardised using SDS (§2.3.1). Mid- parental height SDS was calculated as the average height SDS of both parents. Where missing, either mother or father height SDS was used. Maternal age of menarche (in years) was reported by the mother in the 1969 survey. A continuous measure was used to indicate the genetic effect of pubertal timing for females, as information was rarely collected for male cohort members (missing data = 65%). Maternal smoking during pregnancy and maternal age at study child’s birth was recorded in 1958. A binary variable was derived (smoker vs. non-smoker). Birth weight was measured in ounces and converted to grams. Gestational age (in days) was reported by the mothers, and converted to weeks. Pre-term was categorised as gestation of < 38 week. Mothers were asked how long they and the study child’s father remained in school in 1958. Mother and father’s duration of schooling were classified as leaving prior to the statutory school leaving age (age 14y born before April 1933, age 15y for those born subsequently) and up to, or over minimum leaving age.

2.4.3 Early childhood factors

Childhood factors obtained at birth or age 7y were used to ensure measures were

recorded prior to, or in concordance with childhood exposure and response measures.

Social class at birth was based on the father’s occupation in 1958, and where missing, was supplemented with information collected in the 7y survey. The Registrar General’s classification was used: professional (I), managerial and technical (II), skilled non- manual (IIInm) and skilled manual (IIIm), semi-skilled (IV) and unskilled (V)

worker350;351. Four broad categories were derived; I & II, III non-manual, III manual, and IV & V. Children whose father was unemployed, sick or from lone-mother households were combined with the last group (IV & V).

Breast fed was reported in 1965 when cohort members were age 7y (not breastfed, breastfed <1 month and breastfed ≥ 1 month). Exclusivity of breastfeeding was not

Major disability (e.g. blindness, deafness, cerebral palsy, hydrocephalus etc.) was recorded by the medical examiner at age 7y. A binary measure (yes/no) was derived.

Body mass index (BMI) at age 7y was derived by using height in cm (converted to meters (m)) and weight (in kilograms (kg)) measured during the 7y medical examination; BMI= weight/ (height * height).

The number of household members and number of rooms in accommodation were reported at age 7y. Number of persons per room was calculated. Overcrowded household was defined as ≥ 1.5 persons per room.

Housing tenure was reported at age 7y and classified as; 1) owner occupied, 2) private rental, 3) council or housing authority rental (social housing) and 4) other

accommodation.

Household amenities were obtained at age 7y. The sole or shared use, or no access to a bathroom, indoor lavatory and hot water supply was established (‘sole use’, ‘shared use’, or ‘no access’). An amenities score (range 0-6) was derived; a score of 6 indicating no access to a bathroom, indoor lavatory and hot water.

2.4.4 Confounding factors for height analyses

The established literature has found that child-to-adult height trajectories are influenced by genetic factors, prenatal exposures and early life socio-economic conditions205;208;213. Previous studies investigating the association between adverse childhood experiences and height growth have adjusted for parental height, birth weight and childhood diet, illness, socio-economic position (SEP) and household overcrowding141;144;152-154;352. In Chapter 5, parental height, maternal smoking during pregnancy, pre-term birth, birth weight, social class at birth, breastfed and major disability, household overcrowding, housing tenure and amenity score at age 7y were considered as confounding factors for the association between adverse childhood experiences and height.

In the 1958 cohort, short parental stature was found to be associated with a history of maltreatment and household dysfunction (Appendices 2.5 – 2.7). A greater proportion of participants whose mother smoked during their pregnancy, who had a low birth

dysfunction, compared to those who did not. Social class at birth, household overcrowding, housing tenure and amenities score were also related to adverse

childhood experiences. A greater proportion of participants not breastfed experienced neglect and were from dysfunctional family backgrounds, than those who were

breastfed. Major disability at age 7y was positively associated with childhood neglect and household dysfunction measures, although it was unrelated to retrospective reports of child maltreatment (Appendix 2.5).

Mid-parental height was positively associated with cohort member’s height at age 7, 11, 16y and adult height and leg length (Appendix 2.8). Participants whose mothers

smoked during pregnancy, who had a low birth weight and were born at < 38 weeks gestation, were, on average, shorter at each age, and had shorter legs in adulthood, compared to those who did not. Low social class, overcrowded households, social housing and few household amenities at age 7y were related to short childhood and adult height and adult leg length. Participants who were breastfed were, on average, taller in childhood, than those who were not breastfed. Major disability at age 7y was significantly related to height at all ages, but not adult leg length.

2.4.5 Confounding factors for pubertal development analyses

Genetic and environmental factors207;228-232, as well as race and early nutrition233-235, have been shown to influence the onset of puberty. Previous studies investigating the association between adverse childhood experiences and puberty have adjusted for ethnicity, childhood SEP, level of parental education, maternal age of menarche, marriage and participants birth, and individual’s BMI and height147;156;158-160;162;238;353-356. In Chapter 6, ethnicity, birth weight, social class at birth, breastfeeding, major

disability, household overcrowding, and maternal age of menarche (females only) were considered as confounding factors for the association between adverse childhood experiences and pubertal development. Level of parental education, maternal age at cohort member’s birth were not associated with markers of pubertal development, therefore were not considered confounding factors. Although birth weight, preterm birth and BMI at age 7y were also not associated with adverse childhood experiences or pubertal development measures (Appendices 2.5-2.7), I examined their effect by

associations, and thus they were not included as confounding factors in the final analysis.

A larger proportion of non-white participants reported child maltreatment and

household dysfunction compared to white participants in the 1958 cohort (Appendices 2.5 - 2.7). Older maternal age of menarche was associated with physical abuse and several forms of neglect at age 7y (e.g. mother unaffectionate and mother hardly read, hardly any outings with parents and mother little interest in education) and household dysfunction measures (e.g. domestic tension and time in care) in females.

There was an ethnic different in pubertal development in both boys and girls. More non-white boys compared to white boys were at advanced stages of testicular

development at age 11y, developed pubic hair earlier, had adult facial hair at age 16y and were age ≤ 12y when their voices broke (Appendix 2.9). A greater proportion of non-white girls had advanced breast development and pubic hair growth at age 11y, and experienced menarche at age ≤ 11y compared to white girls (Appendix 2.10). For boys, low social class at birth was associated with late testicular and pubic hair development, but early age of voice change. For girls, low social class at birth was related to early and late age of menarche. Overcrowded household at age 7y was related to late pubertal development in both genders. For all markers of pubertal development in girls, late developers had an older mean maternal age of menarche compared to intermediate developers, whilst early developers had a younger mean maternal age of menarche.