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Parental and Early Childhood Predictors of Persistent Physical Aggression in Boys From Kindergarten to High School
Daniel S. Nagin, PhD;
Richard E. Tremblay, PhD
Arch Gen Psychiatry. 2001;58:389-394.
ABSTRACT
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Background In a prior study, we identified 4 groups following distinct developmental
courses, or trajectories, of physical aggression in 1037 boys from 6 to 15
years of age in a high-risk population sample from Montréal, Québec.
Two were trajectories of high aggression, a persistently high group and a
high but declining group. The other 2 trajectories were a low group and a
moderate declining group. This study identified early predictors of physical
aggression trajectories from ages 6 to 15 years.
Methods In this study, logistic regression analysis was used to identify parental
and child characteristics that distinguished trajectory group membership.
Results For boys displaying high hyperactivity and high opposition in kindergarten,
the odds of membership in the 2 high aggression groups were increased by factors
of 3.0 (95% confidence interval [CI], 2.0-4.3) and 2.7 (95% CI, 1.9-3.8),
respectively, compared with boys without these risks. Counterpart odds ratios
for the risk factors of mothers' teen-onset of parenthood and low educational
attainment were 1.6 (95% CI, 1.1-2.2) and 1.8 (95% CI, 1.3-2.4), respectively.
Only the maternal characteristics distinguished between the trajectory of
persistently physical high aggression and the trajectory starting high but
subsequently declining. For the 2 maternal risk factors combined, the odds
ratio of persisting in high level physical aggression was 9.4 (95% CI, 2.9-30.4).
Conclusions Kindergarten boys displaying high levels of opposition and hyperactivity
are at high risk of persistent physical aggression. However, among kindergarten
boys who display high levels of physical aggression, only mothers' low educational
level and teenage onset of childbearing distinguish those who persist in high
levels of physical aggression.
INTRODUCTION
YOUTH WHO persistently violate the basic rights of others are classified
in the DSM-IV1 as having
conduct disorder (CD). Four categories of behavior can lead to a CD diagnosis:
physical aggression, property destruction, deceitfulness or theft, and serious
rules violations. However, youth with childhood-onset CD (DSM-IV, CD before age 10 years) have been found to exhibit more physical
aggression than those with a later age of onset.2
This study reports on the parental and early childhood predictors of
the developmental course of physical aggression, hereafter called a developmental trajectory. It is a follow-up to our prior
study3 of the same 1037 boys, from low socioeconomic
neighborhoods in Montréal, Québec, in which we identified 4
distinctive developmental trajectories of physical aggression from ages 6
to 15 years: a chronic physical aggression trajectory
(4%) composed of boys displaying persistently high levels of physical aggression;
a high level declining trajectory (28%) composed
of boys who displayed a high level of physical aggression in kindergarten
but whose aggression thereafter declined; a moderate level
declining trajectory (52%) composed of boys who displayed modest physical
aggression in kindergarten but whose aggression thereafter declined to virtual
cessation; and a low trajectory (17%) composed of
boys who rarely displayed physical aggression. Similar trajectories were observed
in 2 large samples from New Zealand and in another sample from Canada.4
All trajectories were basically stable or declining from age 6 years
and older. Self-reported physical violence, theft, and serious delinquency
at age 17 years and court-recorded infractions before age 18 years all increased
sequentially from the low aggression to the chronic aggression groups. These
results suggest that children with the highest levels of physical aggression
in kindergarten were most prone to follow the path of chronic violence. Physical
aggression problems that lead to adolescent violence develop before school
entry,5, 6, 7, 8, 9, 10
and a search for risk factors should target prenatal, perinatal, and early
childhood factors.11
This study aimed to identify variables established early in the boys'
lives that (1) differentiated the chronic and high declining aggression groups
from the more normative, low level, and moderate level declining groups and
(2) discriminated the high (but) declining group from the chronic group. The
latter comparison is of special interest because predictors of the cessation
of violence have not been adequately investigated.
Two categories of variables were examined, characteristics of the parents
and of the boy. Parental characteristics included low educational attainment,
first becoming a parent as a teenager, low socioeconomic status (SES), and
separation or divorce before the boy's sixth birthday. Based on prior research,12, 13, 14, 15, 16, 17, 18, 19, 20, 21
each of these is predicted to increase the risk of a child's membership in
the 2 high physical aggression trajectories compared with the 2 lower level
groups. Except for the separation and divorce variables, all parental risk
factors were established before the boy's birth. However, because genetically
informative data were not available, we cannot determine whether their association
with the boy's behavior is due to shared genes or environmental conditions.
Regarding the boy himself, based on research showing that cognitive22, 23, 24 and behavioral25, 26, 27, 28 characteristics
are associated with physical aggression, a child's low IQ, high hyperactivity,
high opposition, and high inattention were expected to increase the risk of
membership in the 2 high aggression trajectories. Also, high anxiety and high
prosocial behavior were included as protective factors.29, 30
SUBJECTS AND METHODS
SUBJECTS
The subjects in this study were part of a longitudinal study that started
in the spring of 1984, when all teachers of kindergarten classes in the 53
schools of the lowest socioeconomic areas in Montréal were asked to
rate the behavior of each boy in their classroom. The mean and median family
income when the boys were age 10 years (1988) was between Can $25 000
and Can $30 000 (US $19 000 and US $23 000) compared with a
median income of Can $44 000 for Canadian couples with children in 1987.31 Eighty-seven percent of the kindergarten teachers
agreed to participate, and 1161 boys were rated. To control for cultural effects,
the boys were included in the longitudinal study only if both of their biological
parents were born in Canada and their parents' mother tongue was French. Thus,
a homogeneous white, French-speaking sample was created. The sample was reduced
to 1037 boys after applying these criteria and eliminating those who declined
to participate and those who could not be located. Informed consent was regularly
obtained from mothers and the youth throughout the study.
Participation rates in follow-up interviews were high but still short
of 100%. The trajectory estimation procedure used to identify the groups,
described in the introduction, is designed to accommodate missing data in
an individual's assessment record.
When in kindergarten, 67% of the boys lived with both of their parents,
24% lived with only their mothers, and 5% lived with their mother and a man
other than their father; the rest lived in other family arrangements. The
mean ± SD age of the parents at the birth of their child was 25.4 ±
4.8 years for the mothers and 28.4 ± 5.6 years for the fathers. The
mean ± SD age at the birth of their first child was 23.8 ± 4.1
years for the mothers and 26.4 ± 5.1 years for the fathers. The mean
± SD number of school years completed by the parents was 10.5 ±
2.8 years for the mothers and 10.7 ± SD years for the fathers. The
mean score on the Canadian socioeconomic index for occupations was 38.15 for
mothers and 39.19 for fathers. This index ranges from 17.81 for the lowest
status to 101.74 for the highest, with a mean ± SD of 42.74 ±
13.28.32 The mean scores for the mothers and
fathers in this sample correspond to jobs such as file clerk, hospital attendant,
or unskilled factory worker.
MEASURES
Parental and Family Characteristics
Interviewers with postsecondary education were trained to interview
mothers by telephone after the initial teacher assessments in kindergarten.
Questions included years of school completed by the mother and the child's
father; their birth dates; the date of birth of each of their children; their
employment status, including the type of job held or type of last job if they
were not presently working; and whether she was living with the child's father.
From these data, the following binary indicator variables were constructed: Teenage mother and teenage father
identify whether the boy's mother and father first became parents as teenagers. Low education mother and low education
father indicate mothers and fathers who completed 9 or fewer years
of schooling. For both sexes, this put them in the lower quartile of the parental
education distribution for the sampled boys. Low SES mother and low SES father define mothers and fathers
who were in the lower quartile of the sample distribution of the Canadian
socioeconomic index of occupations for each sex. Not intact identifies natural parents who separated or divorced before the boy's
sixth birthday.
Behavior Ratings
Physical aggression, opposition, hyperactivity, inattention, anxiety,
and prosocial behavior were rated by the teacher most knowledgeable about
the child at ages 6 and 10 to 15 years using the Social Behavior Questionnaire.33 The teachers were given a list of statements describing
children's behavior and were asked to rate how often (often, 2; sometimes,
1; never, 0) the child behaved according to the description. Physical aggression
at ages 6 and 10 to 15 years was assessed with 3 items: fights with other
children; kicks, bites, or hits other children; and bullies or intimidates
other children (Cronbach  = 0.78-0.87; mean, .84). Opposition in kindergarten
was assessed with 5 items: does not share materials, irritable, disobedient,
blames others, and inconsiderate (Cronbach = 0.84). Hyperactivity
in kindergarten was assessed with 2 items: squirmy, fidgety; and does not
keep still (Cronbach = 0.87). Inattention in kindergarten was assessed
with 2 items: inattentive and has poor concentration (Cronbach = 0.74).
Anxiety in kindergarten was assessed with 5 items: is worried; tends to do
things on his own, solitary; miserable, unhappy; fearful, afraid of new things;
and cries easily (Cronbach = 0.76). Prosocial behavior in kindergarten
was assessed with 10 items: tries to stop quarrels, invites bystanders to
join in, tries to help someone who is hurt, helps pick up things someone else
dropped, praises work of less able children, shows sympathy toward someone
who made a mistake, helps children having difficulty with a task, helps children
who are sick, comforts crying or upset child, and helps clean up mess made
by someone else (Cronbach = 0.92).
From these measurements, the binary variables high
hyperactivity, high opposition, high inattention, high anxiety, and high prosociality were constructed. Each identifies boys
in about the upper quartile of each of the respective sample distributions.
Verbal IQ Assessment
Verbal IQ was assessed at age 13 years with the Sentence Completion
Test.34 Intelligence quotient is generally relatively
stable from kindergarten to adolescence.35 The
assessment at age 13 years should give an estimate of the relative ranking
of the boys on cognitive performance when they were in kindergarten. A correlation
of 0.67 was found when the Sentence Completion Test score at age 13 years
for a subsample of subjects (n = 80) was correlated with an IQ assessment
at age 10 years using the vocabulary and block design subtest of the Wechsler
Intelligence Scale for Children–Revised. Low IQ
identifies boys in the lower quartile of the sample verbal IQ distribution.
DATA ANALYSIS
The analysis was designed to identify risk and protective factors distinguishing
the 4 trajectory groups described in the introduction. The technical details
of the semiparametric, mixture model used to identify these groups are described
elsewhere.36 One key output of the model, called
the posterior probabilities of group membership,
was central to the analyses reported herein. For each individual in the sample,
these probabilities estimate the probability of the individual's belonging
to each trajectory group. For example, consider an individual who persistently
received high physical aggression ratings by teachers. For this individual,
the posterior probability estimate of his belonging to the low trajectory
group would be near zero, whereas the estimate of his belonging to the chronic
group would be high. Individuals were assigned to the group with the largest
posterior probability estimate. This is the group that best conforms to a
boy's observed behavior. These group membership designations form the point
of departure for this analysis. Multinomial and binary logistic regression
analyses were performed to identify parental and early childhood characteristics
that distinguish membership in the various trajectory groups.
The analysis proceeded in 2 stages. First,  2 tests of
joint significance were used to identify parental and early childhood risk
and protective factors that were statistically significant in distinguishing
membership across the 4 trajectory groups. The second-stage analysis examined
the capacity of these statistically significant risk and protective factors
to distinguish membership in the 2 lower physical aggression trajectories,
the low and the moderate declining groups, from the 2 high physical aggression
trajectories, the chronic and the high declining groups. We also examined
the capacity of these factors to distinguish between the chronic and the high
declining groups alone. Results of the analysis were substantively identical
for models in which the risk variables were entered in their nonbinary form.
Throughout, 2-tailed tests of significance were conducted for = .05.
RESULTS
Table 1 reports the prevalence
of each risk and protective factor by trajectory group. The teenage mother,
low maternal education, household not intact, low IQ, high hyperactivity,
high opposition, high inattention, and high prosociality variables significantly
distinguished group membership. Furthermore, except for the prosociality factor,
their prevalence increases sequentially from the low group to the chronic
group. This accords with expectations. For the prosociality variable, the
pattern is reversed, which is also as expected. However, neither mother's
nor father's low SES was significant, nor were the early parenthood or low
education factors for the father significant. Thus, in toto, the fathers'
characteristics did not distinguish trajectory group membership. In addition,
child high anxiety was no significant predictor.
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Table 1. Parental and Child Characteristics by Trajectory Group*
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The second column of Table 2
reports the results of the multivariate logistic regression analysis aimed
at distinguishing boys (69%) who follow the 2 low physical aggression trajectory
groups from their counterparts in the 2 high trajectory groups. Entries in
the table are the odds ratios for each of the risk or protective factors found
to be significant in Table 1. The
results show that the magnitudes of the impacts are substantial. Maternal
risk factors of low education and teenage motherhood increased the odds of
a high aggression trajectory by 77% and 57%, respectively. The largest impacts
were for the behavioral and cognitive risk factors of the child himself. Low
verbal IQ was associated with a nearly 2-fold increase in the odds of a high
aggression trajectory, while high hyperactivity and high opposition increased
the odds by nearly 3-fold. In contrast, prosociality decreased the odds of
high aggression by more than half. In combination, the 2 largest risk factors,
high hyperactivity and high opposition, increased the odds of high aggression
by a factor of 8.0 (95% confidence interval [CI], 5.2-12.2). Only the high
inattention risk factor was statistically insignificant in this multivariate
model.
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Table 2. Predictors of High Physical Aggression and of Chronic Physical
Aggression*
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Odds ratio calculations do not measure the absolute risk of the behavior
for persons with and without the risk factor. As would be expected from the
odds ratio analysis, the highest rate of absolute discrimination was for the
high hyperactivity risk factor; 63% of the high hyperactivity boys belonged
to a high aggression group, compared with only 23% of the boys without this
risk factor. Still, even for hyperactivity, it is clear that no single risk
factor is decisive in predicting trajectory group membership.
Only 2 factors distinguished between the high level declining and chronic
trajectories, and both measured maternal characteristics. The low education
and teenage mother risk factors increased the odds of following the chronic
trajectory by factors of 3.2 (95% CI, 1.4-7.4) and 2.9 (95% CI, 1.3-6.4),
respectively. In combination, they increased the odds by a factor of 9.4 (95%
CI, 2.9-30.4). Thus, whereas the individual-level psychological and cognitive
variables have the largest impacts on the risk of a boy following a high vs
low physical aggression trajectory, the mother-related variables seem to be
most important in distinguishing the boys who desist from high levels of physical
aggression from those who persist in it. Still, these 2 maternal characteristics
fall well short of definitively identifying the chronic group from the high
declining group. For this subset of boys, 9.1% were in the chronic group.
For those with the low education and teenage mother risk factors, the percentage
belonging to chronic groups is 14.0% and 15.7%, respectively. For those with
both these risk factors, 21.3% are in the chronic group.
COMMENT
The focus of this analysis was establishing risk factors for the various
trajectories of physical aggression in boys from 6 to 15 years of age. A host
of risk factors were identified that distinguish the 2 low physical aggression
trajectories from the 2 high trajectories. Concerning these risk factors,
2 findings stand out. The most powerful predictors of membership in a high
aggression trajectory group were high levels of hyperactivity and opposition
assessed in kindergarten. Individually, these risk factors increased the odds
of membership in a high physical aggression trajectory by about a factor of
3. In combination, the increase is more than 9-fold. Individually, these risks
are comparable in magnitude to the impact of high serum cholesterol levels
on risk of coronary heart disease.37 In combination,
they far exceed it.
A second prominent finding concerned the predictive power of parental
characteristics: only characteristics of the mother have predictive power.
Whereas teen onset of parenthood and low educational attainment for mothers
were significant predictors of their son's high aggression, these characteristics
in the father had no predictive power.
The prominent predictive power of maternal characteristics again revealed
itself in the analysis of factors that distinguished between the 2 high physical
aggression groups, the high declining and the chronic trajectories. Here,
we were concerned with identifying characteristics that distinguish the modestly
large fraction (28%) of boys who start off displaying high levels of physical
aggression but subsequently desist, from the small but prominent group (4%)
of boys who continue their physical aggression unabated. Only 2 such characteristics
were identified, mother's low educational attainment and teenage onset of
childbearing. The odds of male offspring of poorly educated teenage mothers
not desisting from a high level of physical aggression at age 6 years are
9.3 times greater than those of their counterparts without such mothers.
Although ours is the first analysis to document that these 2 maternal
characteristics distinguish persistence in from desistance of chronic physical
aggression, there is a large body of evidence linking teen onset of childbearing
with a litany of unfavorable behaviors and outcomes for the offspring. These
include CD and other problem behaviors in childhood,38, 39
delinquency and school dropout in adolescence,40, 41, 42
and criminality as an adult.43
This study does not explain why teen onset of motherhood and low maternal
education are risk factors for chronic physical aggression in a mother's offspring.
These 2 maternal factors are likely markers of maternal problem behaviors
and circumstances that give rise to bad outcomes for her offspring rather
than the causes per se. For example, more-aggressive young women are more
likely to become teen mothers, to drop out of school, and to be unresponsive
parents. There is also evidence that women who begin childbearing early are
more likely to use harsh and erratic discipline. These mothers may also be
more prone to birth complications, which have been shown to be related to
subsequent conduct problems in the child.44, 45
These findings suggest that the mothers themselves may be the agents of the
intergenerational transfer of chronic physical aggression. The results are
not nearly definitive about the specifics of the transfer mechanism in terms
of the separate and interactive roles of biology, parenting practice (including
the father), and the larger social environment. Notwithstanding, they suggest
that the intergenerational transfer mechanism may have profound consequences
for the child and society.
There are limitations to this study. Key findings pertain to 2 maternal
characteristics, early onset of childbearing and low educational attainment,
which are probably markers of causal factors active long before the study
began (ie, when the boys were attending kindergarten). We have speculated
that adolescent mothers with low educational attainment tend to lack the skills
needed to create a context in which children learn to regulate physical aggression.
Future research should directly test this contention. Such studies should
also control for genetic, prenatal, and perinatal factors, which have all
been associated with the development of antisocial behavior.44, 45, 46, 47, 48
Understanding the causal processes will have important implications for designing
interventions aimed at preventing chronic physical aggression. Finally, the
trajectories of physical aggression studied herein end in mid-adolescence.
As such, they may not reflect behavioral changes resulting from important
developmental shifts in biological and contextual factors during the transitions
into and out of adolescence. Gains in physical size and strength accompanying
puberty, coupled with reductions in parental and other adult supervision and
increases in the amount and importance of peer interaction, could all trigger
sudden increases in violence not captured in these analyses. Moreover, potential
changes in the manifestation of these behaviors that may accompany development
might not be adequately captured by the school-based assessments of teachers.
Thus, it would be desirable to replicate our analyses with alternative, non–school-based
measures extending later into life.
Our findings have implications for clinical practice and public policy.
Special priority should be given to identifying and helping children at greatest
risk for chronic physical aggression. By the end of kindergarten, there are
several powerful predictors of such risk—high opposition and high hyperactivity,
low IQ, family breakup, teen motherhood, and low maternal education. Interventions
for these children have been shown to have some long-term impact.49, 50, 51
Furthermore, even earlier identification and intervention may have a
greater preventive impact because learning to regulate physical aggression
appears to start in the first few years after birth. Maternal characteristics
are potentially powerful markers for identifying children at greatest risk
of not learning such regulation. The findings suggest that special attention
should be given to targeting women with low education and those who begin
childbearing early. Prepartum and postpartum programs for reducing risk-taking
behaviors (eg, use of cigarettes, alcohol, and other drugs) and improving
parenting skills should be targeted toward these high-risk mothers. High-quality
day-care programs should also be administered to their children. There is
a growing body of persuasive experimental evidence demonstrating that such
programs have salutary impacts on targeted children's level of behavior problems,
success in school, and degree of antisocial behavior during adolescence and
early adulthood.52, 53, 54, 55, 56, 57
However, it is important that the mother's participation in these programs
be fully voluntary, not only to gain her full cooperation but also out of
recognition that most children of poorly educated women who begin childbearing
early are not destined to follow a trajectory of chronic physical aggression.
Although our results show that the risk of such children being persistently
violent is greatly heightened by these maternal risk factors, most of them
do not follow this developmental trajectory.
AUTHOR INFORMATION
Accepted for publication October 11, 2000.
This study was made possible by grant RS-2300 from Conseil québécois
de la recherche sociale and grant 01CE0169 from Le Fonds pour la formation
de chercheurs et l'aide à la recherche funding agencies, Québec
City, Québec; grant 6605-4391-102 from National Health Research and
Development Program, Ottawa, Ontario; Social Sciences and Humanities Research
Council of Canada, Ottawa, funding agencies; the Molson Foundation, Montréal,
Québec; the National Consortium on Violence Research, Pittsburgh, Pa;
and grants SBR-9511412 and SES-9911370 from the National Science Foundation,
Arlington, Va.
We thank Thomas Detre, MD, and Frederick Rivara, MD, for their input;
Hélène Beauchesne, MSc, and Lucille David, MSc, for supervising
the data collection; Lyse Desmarais-Gervais, MSc, Pierre McDuff, MSc, and
Muriel Rorive, BSc, for managing the data bank; and Katia Maliantovitch, MEd,
for revising the manuscript.
From H. John Heinz III School of Public Policy and Management, Carnegie
Mellon University, Pittsburgh, Pa (Dr Nagin); and Research Unit on Children's
Psychosocial Maladjustment, University of Montréal, Montréal,
Québec (Dr Tremblay).
Corresponding author and reprints: Daniel S. Nagin, PhD, 2105 Hamburg
Hall, Carnegie Mellon University, Pittsburgh, PA 15213 (e-mail: dn03{at}andrew.cmu.edu).
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