A new study from Brazil has shed light on the complex origins of attention-deficit hyperactivity disorder, commonly known as ADHD. Researchers discovered that both a person’s genetic predisposition for ADHD and experiences of childhood maltreatment independently increase the likelihood of experiencing ADHD symptoms in adulthood. Furthermore, the study suggests that genetic factors that raise the risk of ADHD may also indirectly increase the risk of a child experiencing maltreatment. The findings were published in Molecular Psychiatry.
Attention-deficit hyperactivity disorder is a condition that affects both children and adults, making it difficult to concentrate, control impulsive behaviors, and manage hyperactivity. It is considered a neurodevelopmental disorder, meaning it stems from differences in brain development. People with ADHD may struggle with organization, following instructions, and sitting still. While ADHD often starts in childhood, for many individuals, the symptoms continue into adulthood, impacting their work, relationships, and overall quality of life.
Scientists have long known that ADHD has a strong genetic component, meaning it tends to run in families. However, environmental factors, particularly negative childhood experiences, are also thought to play a significant role. Childhood maltreatment, which includes physical, emotional, and sexual abuse, as well as neglect, is a serious global problem with wide-ranging negative consequences for a child’s development and later life. Previous research has shown a connection between childhood maltreatment and an increased risk of developing ADHD.
To gain a deeper understanding of this connection, researchers in Brazil decided to investigate how genetic predisposition and childhood maltreatment might interact to influence ADHD symptoms in adulthood. They wanted to explore if genes and environment act independently, or if they influence each other in complex ways. Specifically, they looked at two possibilities: gene-environment interaction and gene-environment correlation.
Gene-environment interaction would mean that the effect of genes on ADHD risk is different depending on whether someone has experienced maltreatment or not. Gene-environment correlation suggests that genes may influence the environment a person experiences. This study is particularly important because most previous research in this area has been conducted in high-income countries, while the rates of childhood maltreatment are often higher in low- and middle-income countries like Brazil.
The researchers used data from a long-term study that has been following a group of individuals born in the city of Pelotas, Brazil, in 2004. This study, known as the 2004 Pelotas birth cohort, initially included nearly all babies born in the city that year. The participants have been examined at various ages throughout their lives. For this particular study, the researchers focused on data collected when the participants were 18 years old.
At age 18, the young adults completed a self-report questionnaire called the Adult Self Report Scale for ADHD. This questionnaire asks about symptoms of inattention, hyperactivity, and impulsivity that are typical of ADHD. Mothers or caregivers also completed a questionnaire called the Strengths and Difficulties Questionnaire, reporting on their child’s behavior, including hyperactivity and inattention. Using both self-reports and reports from mothers provided a more comprehensive picture of ADHD symptoms.
To assess genetic predisposition for ADHD, the researchers used genetic information collected from saliva samples taken when the participants were 6 years old. They calculated an ADHD genetic score for each participant. This score, called a polygenic score, is based on the combined effect of many common genetic variations across the entire genome that have been linked to ADHD in large previous studies. A higher score indicates a greater genetic likelihood of developing ADHD.
Childhood maltreatment was evaluated using the Parent-Child Conflict Tactics Scale. This questionnaire was administered to mothers or caregivers when the children were 6 and 11 years old. It asks about parenting behaviors that could be considered physical and psychological maltreatment, such as yelling, shaking, or hitting a child. The researchers used the average maltreatment score from ages 6 and 11 to represent overall childhood maltreatment experience.
After gathering all this data, the researchers used statistical methods to analyze the relationships between genetic scores, childhood maltreatment, and ADHD symptoms at age 18. They considered various factors that could influence ADHD, such as sex, skin color, birth weight, intelligence quotient, maternal smoking during pregnancy, maternal education, family income, household size, and maternal depression, to ensure their findings were not due to these other factors.
The study revealed several important findings. First, the researchers confirmed that the ADHD genetic score was associated with ADHD symptoms in adulthood, as reported by both the young adults themselves and their mothers. This means that individuals with higher genetic scores were more likely to show ADHD symptoms at age 18.
Second, they found that childhood maltreatment was also linked to ADHD symptoms in adulthood. Individuals who experienced more maltreatment in childhood, according to reports from their mothers or caregivers, were more likely to report ADHD symptoms themselves and were also rated as having more symptoms by their mothers at age 18. This was true for both overall maltreatment and specifically for psychological and physical aggression, including corporal punishment.
Third, and interestingly, the study found that the ADHD genetic score was also associated with childhood maltreatment scores. This suggests that children with a higher genetic predisposition for ADHD were more likely to experience maltreatment.
To explore this further, the researchers conducted additional analyses to investigate the mechanism behind this link. They found that ADHD symptoms observed at age 6 appeared to act as a mediator between genetic predisposition and later maltreatment. In other words, children with a higher genetic risk for ADHD might exhibit more ADHD symptoms early in life, and these early symptoms might, in turn, increase the likelihood of experiencing maltreatment. This suggests what is known as an evocative gene-environment correlation, where a child’s genetic traits influence how others, like parents, behave towards them.
However, the study did not find evidence for gene-environment interaction. This means that the effect of genetic predisposition on ADHD symptoms was not significantly different for those who experienced maltreatment compared to those who did not. Genes and environment appeared to have independent effects on ADHD risk.
The researchers acknowledged some limitations of their study. Although the Pelotas birth cohort is a valuable resource, some participants were lost to follow-up over time, which could introduce some bias. The measurement of maltreatment relied on reports from mothers or caregivers and only captured maltreatment at ages 6 and 11, potentially missing maltreatment at other times or reported by other individuals. Also, for ethical reasons, the study did not specifically investigate more severe forms of maltreatment, such as sexual abuse. Furthermore, genetic scores developed primarily in European populations may not be as accurate in predicting ADHD risk in diverse populations like those in Brazil.
Despite these limitations, the study has several strengths, including its long-term prospective design, the use of validated questionnaires, and the inclusion of both self and mother reports of ADHD symptoms. The fact that the study was conducted in a middle-income country like Brazil, where rates of childhood adversity can be high, adds to the importance of the findings.
The study, “Gene – maltreatment interplay in adult ADHD symptoms: main role of a gene–environment correlation effect in a Brazilian population longitudinal study,” was authored by Luciana Tovo-Rodrigues, Laísa Camerini, Thais Martins-Silva, Marina Xavier Carpena, Carolina Bonilla, Isabel Oliveira Oliveira, Cristiane Silvestre de Paula, Joseph Murray, Aluísio J. D. Barros, Iná S. Santos, Luis Augusto Rohde, Mara Helena Hutz, Julia Pasqualini Genro, and Alicia Matijasevich.