Developmental Course and Risk Factors of Physical Aggression in Late Adolescence

Physical aggression in childhood and adolescence represents a major clinical and public health concern. Thus, in a population study in the United States in 2017, 23.6% of high school students (grade 9 to 12) reported participating in a physical fight in the preceding year [1]. At the global level, interpersonal violence is the fourth leading cause of death among adolescents aged 15 to 19, with a 5.5% mortality rate [2]. Physical aggression and delinquency can have long lasting negative consequences for the perpetrators and the victims. Perpetrators of physical aggression are shown to have an increased risk for later physical violence and crimes [3, 4], school drop-out [4, 5], substance abuse [5], future mental, and physical health problems and economic difficulties [6]. Victims, in addition to the physical injuries, have an increased risk of mental health problems, educational and economic difficulties, involvement in crimes [7], and externalizing problems [8].

From a developmental perspective, the type and prevalence of physically aggressive behavior change with age. In typically developing youth, the prevalence of physical aggression peak at around age 3 and decrease from childhood to adolescence, as children acquire more advanced cognitive, social and language skills, and learn to regulate emotions and use alternative conflict resolution strategies [9‐11]. Some children continue to display high levels of physical aggression throughout their childhood and adolescence [12‐14]. Nagin and Tremblay [3], in a sample of 1037 boys followed from age 6 to 15 years old, identified four trajectories of physical aggression: chronic/persisting aggression (4%), high near-desisting (28%), moderate desisting (52%) and no physical aggression trajectory (17%). Since this seminal study, these developmental trajectories of physical aggression have been well replicated in other studies [10, 13, 15, 16], although the number of trajectories and number of children in each category varies as a function of sample characteristics and aggression measurements.

Empirical studies have identified multiple correlates and risk factors of physical aggression in adolescence [17, 18]. Both girls and boys can initiate aggressive behavior, but boys display more physical aggression than girls [9, 11, 13, 16]. Mental health disorders are associated with physical aggression, most notably disruptive behavior disorders and attention-deficit/hyperactivity disorder (ADHD), but also anxiety and depression [19‐21]. Longitudinal studies have found ADHD symptoms to predict physical aggression [3, 5, 22], physical aggression to predict depression [23, 24], and anxiety to protect against aggression [25]. Physical aggression is also associated with academic difficulties [26, 27], and low self-esteem [28]; however, some research indicates that high self-esteem may also be associated with physically aggressive behavior [29].

Regarding social factors, family characteristics including single-parent household and divorced parents have been associated with elevated risk for physical aggression [10, 16, 17, 30]. While some studies present evidence that having divorced parents is a risk factor for later physical aggression [31, 32], others do not [10]. Peer factors, such as peer rejection, bullying, and loneliness, commonly co-occur with physical aggression in adolescence [33‐36], and bullying victimization in adolescence has been linked to later violence [37].

During adolescence, developmentally normative changes in social relationships, including decreasing parental supervision and increasing influence of peers, may also elevate risk for aggression [31, 38]. Similarly, engagement in new, risky behaviors may impact aggressive behavior. For example, increases in alcohol drinking and intoxication, drug use and smoking have been associated with physical aggression in adolescence [5, 39‐42]. In a study on alcohol consumption and aggression (verbal and physical) within a large sample of college students (n = 6282), a direct relationship between alcohol and aggression was found, indicating that increased alcohol use increases risk for aggression [43].

Compared to the large number of studies examining risk factors of physical aggression in childhood and adolescence, much less is known about the risk factors of physical aggression in late adolescence, during the important transition to adulthood. Longitudinal studies of aggression that follow children into young adulthood often focus on broad categories of delinquency [44, 45], rather than specific acts of physical aggression such as fighting. However, studies on physical aggression in childhood and adolescence have shown that physical aggression is a specific and separate dimension of disruptive behavioral problems [46], and physical aggression in late adolescence and young adulthood can also occur outside of delinquency trajectories, for example in relation to intimate partner violence [47]. In addition, few studies have examined the risk factors and courses of physical aggression from early to late adolescence. During this time, adolescents are faced with important developmental tasks, such as identity formation, separating from parents, and developing autonomy and intimacy with others [48, 49] at the same time as parents’ supervision decreases. Although physical aggression generally declines with increasing age, the specific characteristics of the adolescent period may place some individuals at risk for the onset of physically aggressive behavior during this developmental phase. Two studies support the trend of increasing physically aggressive behavior over the course of adolescence [15, 31]. However, one of these studies did not examine risk factors of courses of aggression, and the other study used a broader measure of aggression that did not distinguish between physical and verbal aggression (e.g., verbal threats).

In this study, we used the data from the Young-HUNT study to examine correlates of physical aggression in early adolescence and examine risk factors for physical aggression during the transition from early to late adolescence. Our results showed that younger age, male sex, anxiety and depression symptoms, loneliness, self-esteem problems, attention problems, reading and writing problems, being bullied, alcohol intoxications, and smoking were all associated with physical aggression in early adolescence. Male sex, having attention problems, and drinking alcohol and smoking cigarettes in early adolescence (mean age 14.5 years) increased the risk of emerging physical aggression in late adolescence (mean age 18.4 years), and only male sex and frequent alcohol intoxications were related to increased risk of continuing physical aggression from early to late adolescence.

Our results concerning correlates of physical aggression in early adolescence were consistent with the literature [17‐20, 28, 39]. While some of the associations attenuated in the fully adjusted model, attention problems, reading and writing problems, being bullied, alcohol intoxication frequency between 1 and 10 times and more than 10 times, and smoking still were significantly associated. The strongest associations were between being drunk more than 10 times, being bullied, and having attention problems and physical aggression. Having divorced parents was the only variable not associated with physical aggression in early adolescence. One possible reason for the attenuating effect of anxiety and depression could be the combining of anxiety and depression problems in the same scale. Previous findings have shown considerable evidence for the association between depression and physical aggression [20], while the association with anxiety is more ambiguous, such that anxiety is shown to be both associated with [21] and protective against aggression [25]. Both high and low self-esteem have been found to correlate with physical aggression [29], which could be an explanation for the attenuated effect of low self-esteem in the fully adjusted, more conservative model. Having friends and participating in social activities are for many the most important part of adolescence. Being bullied has a strong association with physical aggression in early adolescence in our study. It could be that the effect of loneliness was attenuated in the fully adjusted model because bullying was a better predictor and accounted for the variance captured by loneliness. Also, experiencing loneliness could be linked to other factors, for example, lack of mutual interests, that are not necessarily associated with bullying or peer rejection and thus do not increase the risk for physical aggression.

The prevalence of physical aggression among adolescents in the present study was 17.4% in early adolescence and 9.1% in late adolescence, which is somewhat lower than the prevalence in an American population sample [1] but still represents a profound clinical and societal challenge. The proportion in the high persistent aggression group (4.9%) resembles high aggression groups in previous studies [3, 10, 15]. However, given differences in sample characteristics and aggression measurements between previous studies and ours, a direct comparison of our aggression courses and those in previous studies is not possible.

Our main goal in this study was to examine if risk factors in early adolescence could help explain why non-aggressive adolescents started displaying physical aggression in late adolescence, and why some adolescents continued with physically aggressive behavior while others desisted. 4.3% of the sample reported onset of physical aggression in late adolescence, which is consistent with previous studies identifying a small group of adolescents with aggressive behavior starting in adolescence [15, 31]. Our results showed that boys with attention problems and substance use (e.g. heavy drinking and smoking) in early adolescence had an increased risk of physical fighting, 4 years later in late adolescence. The predictive effect of having attention problems attenuated in the fully adjusted model, but the effect of male sex, alcohol drinking (intoxications 1–10 times), and smoking remained strong. The predictive effect of delinquent behaviors, such as drinking and smoking, early in adolescence on physical aggression starting in late adolescence could be understood in the context of the specific characteristics of the adolescent period [15, 48, 49]. During the transition to adulthood, the influence of friends typically increases, while parental supervision decreases. Drinking alcohol and smoking in early adolescence could be associated with involvement with deviant friends, providing an accepting environment for norm-breaking behavior, which over time can increase the risk for behaviors such as physical fighting. ADHD symptoms have been shown to have a predictive effect on physical aggression [3, 5, 22]. Our results indicate that attention problems are of some relevance, but they are not an important predictor in distinguishing between non-aggressive behavior and physical aggression starting in late adolescence. It could be that the emergence of physical aggression in late adolescence is accounted for by alcohol use and smoking.

Knowing the negative consequences for the victims and the perpetrators, it is important to be able to identify the group with highly persistent aggressive behavior and know what separates this group from the other courses of aggression. In terms of developing interventions aimed at stopping physically aggressive behavior in adolescence, it is especially interesting to know what separates persistence from desistance of physical aggression. Our results showed that male sex and frequency of alcohol intoxication increased the risk of persistent aggressive behavior during adolescence but that the effect of male sex was attenuated when all of the risk factors were included, leaving heavy drinking as the sole important predictive factor separating adolescents who followed a course of persistent aggression from those who followed a course of desisting aggression. Results indicated a dose–response relationship between alcohol and aggression, as shown in previous research [43]. Recent studies on alcohol consumption have shown a declining trend in alcohol drinking among adolescents [61], which could result in declining physical aggression. However, it is unclear if the declining trend applies to all levels of consumption, especially the heavy drinking.

Results from this study regarding sex and overall age trends in physically aggressive behavior are consistent with previous studies [6, 9‐11, 13, 15, 16]. Our results show that the prevalence of physical aggression is higher in boys than in girls and that more boys than girls are in the persistent aggression group. The prevalence of physical aggression decreased from early adolescence to late adolescence, and lower age was associated with an increased risk of physical aggression in the cross-sectional analysis. These findings are in line with previous studies showing a decrease in physical aggression with increasing age, thereby supporting the understanding that physical aggression is a normative part of children’s development and that as children get older and mature their physical aggressive behavior decreases [11].

Physical aggression during childhood and adolescence is associated with many different risk factors, and the risk factors vary for different developmental periods. For interventions in adolescence to be effective, they need to target the relevant risk factors. Our results showed that heavy alcohol drinking in early adolescence is concurrently and longitudinally associated with aggressive behavior. Excessive alcohol drinking predicted the onset of aggressive behavior starting in late adolescence and separated persistent from desisting physical aggressive behavior over the later adolescent years. These findings highlight the need for early screening and assessment of alcohol drinking in adolescence. Taken together, the results indicate the need to identify adolescents at risk, with a specific focus on boys, as early as possible. Prevention efforts, in general, could focus on reducing alcohol drinking and smoking in early adolescence, while prevention efforts in schools could focus on facilitating mastery for students with attention and learning difficulties and on preventing bullying.

The major strength of the current study is the longitudinal design, the large population-based sample with a high response rate, and a dataset that allows for investigation of a wide array of correlates. In this study, we chose to examine the development of physical aggression from early to late adolescence. Examining the same adolescents at two timepoints (YH1 and YH2), four years apart, provides important information on the different courses of physical aggression in adolescence and corresponding risk factors.

A limitation of the study is that the measurement for physical aggression is based on only one item assessing engagement in physical fights. However, one- item measurements of physical aggression have been used in other studies [62‐64], and while more nuanced measures of aggression are commonly used in clinical populations with smaller samples, they are less used in large population-based surveys. Other forms of physical aggression were not captured in this study, which may have resulted in the lower prevalence of physical aggression in this study and affected the associations we observed. Another limitation is shared method variance, as both the dependent variable physical aggression and the risk factors were measured by self-report questionnaire and completed by the same informant. Also, we had only two timepoints (YH1 and YH2) to base the aggression courses on. Additional timepoints would allow us to chart different aggression trajectories using more sophisticated, person-centered approaches (e.g., latent class growth analysis) to better probe the individual differences in aggression trajectories. Previous research suggests that SES is associated with physical aggression [10]. Our analyses adjusted for sex and age but not SES, which is a limitation. The questionnaire includes a broad range of physical and mental health variables and lifestyle factors. None of the scales or items have diagnostic precision but assess symptoms and problems. Some of the scales are shortened versions of original screening instruments, although shown to be reliable in previous studies [57, 58].

We identified several strong correlates of physical aggression in early adolescence including male gender, attention problems, reading and writing problems, being bullied, intensive alcohol drinking, and smoking. Different risk factors were predictive of different courses of physical aggression. For example, being male and engaging in heavy drinking and smoking during young adolescence were important predictors of physically aggressive behavior starting in late adolescence, while intensive alcohol drinking was predictive of persistent versus desisting use of physical aggression from early to late adolescence. The findings provide insights that can inform the development of preventative measures and therapeutic interventions targeting physical aggression in adolescence.