Introduction
Autism is a lifelong condition that affects how people communicate and interact with others. Autism is also associated with a range of behavioral and cognitive traits, such as social difficulties, repetitive behaviors, and sensory sensitivities. These traits can vary in severity and frequency across individuals and across the lifespan.
But what causes autism and its related traits? How do genetic and environmental factors influence the development and persistence of autism and autistic traits from childhood to adulthood? These are some of the questions that a team of researchers from Sweden and Norway tried to answer in their recent study published in the Journal of Child Psychology and Psychiatry.
What did the researchers do?
The researchers used two different methods to examine the role of genes and environment in autism and autistic traits across different ages.
First, they used a twin method to estimate the heritability of clinically diagnosed autism in childhood and adulthood. Heritability is the proportion of variation in a trait that can be explained by genetic factors. The researchers identified 485 individuals with a clinical autism diagnosis from a large Swedish twin registry that included 23,849 twin pairs born between 1959 and 2010. They compared the rates of autism diagnosis between identical twins (who share 100% of their genes) and fraternal twins (who share 50% of their genes on average) to estimate how much of the difference in autism diagnosis was due to genetic factors.
Second, they used a longitudinal twin method to assess the stability and change of autistic traits from childhood to adulthood. Autistic traits are measured by questionnaires that capture the degree of social, communication, and behavioral difficulties that people experience. The researchers used data from 1,348 twin pairs who participated in a longitudinal study of child and adolescent development in Sweden. The twins completed three different scales of autistic traits at four time points: ages 8-9, 13-14, 16-17, and 19-20. The researchers used a statistical model to estimate how much of the variation and correlation in autistic traits across time and scales was due to genetic, shared environmental (factors that make twins similar to each other), and nonshared environmental (factors that make twins different from each other) influences.
What did the researchers find?
The researchers found that:
- Autism heritability was very high and comparable from childhood (96%) to adulthood (87%). This means that genetic factors play a major role in the development and persistence of autism across the lifespan.
- Autistic traits were moderately stable from childhood to adulthood, with correlations ranging from 0.35 to 0.61. This means that people who had higher or lower levels of autistic traits at one age tended to have similar levels at another age, but there was also some variability and change over time.
- The heritability of autistic traits varied from 52% to 71% across different ages and scales. This means that genetic factors explained about half to two-thirds of the variation in autistic traits, while the rest was due to environmental factors.
- There were both stable and new genetic influences on autistic traits from childhood to adulthood. This means that some of the same genes that affected autistic traits at one age also affected them at another age, but there were also some genes that had age-specific effects.
- There were unique nonshared environmental influences on autistic traits at each age. This means that there were factors that made twins different from each other in their autistic traits that were not shared by their co-twin or by other twins at the same age.
What does this mean?
This study provides new insights into the genetic and environmental factors that shape autism and autistic traits across the lifespan. The findings suggest that:
- Autism is largely influenced by genetic factors that are stable from childhood to adulthood. This implies that autism is not likely to be caused by environmental factors that occur later in life, such as infections, toxins, or stress. However, environmental factors may still affect the expression and severity of autism symptoms in different individuals and contexts.
- Autistic traits are moderately stable but also change over time. This implies that autistic traits are not fixed or static, but rather dynamic and responsive to developmental and environmental changes. This also suggests that autistic traits are not necessarily indicative of autism diagnosis, as some people may have high levels of autistic traits without meeting the clinical criteria for autism, and vice versa.
- Genetic and environmental factors interact and vary in their effects on autistic traits across different ages and domains. This implies that autism and autistic traits are complex and multifaceted phenomena that are influenced by a combination of factors that may have different effects at different stages of development and in different aspects of behavior and cognition.
What are the implications and limitations of this study?
This study has several implications and limitations that should be considered when interpreting and applying the results. Some of the implications are:
- The study provides evidence for the importance of genetic studies in autistic adults, as most of the previous research has focused on children and adolescents. This may help to identify new genes and biological pathways that are involved in autism and autistic traits in adulthood, and to understand how they relate to those in childhood.
- The study also highlights the need for longitudinal studies that follow individuals with autism and autistic traits from childhood to adulthood, and that measure different aspects of their behavior, cognition, and functioning. This may help to understand how autism and autistic traits change over time, and what factors influence their development and outcomes.
- The study also suggests that interventions and supports for autistic individuals should be tailored to their age, needs, and preferences, and that they should take into account the genetic and environmental factors that affect their autistic traits. This may help to optimize the effectiveness and acceptability of the interventions and supports, and to enhance the well-being and quality of life of autistic individuals and their families.
Some of the limitations are:
- The study used a twin design, which assumes that identical and fraternal twins are equally similar in their exposure to environmental factors. This may not always be the case, as some environmental factors may affect twins differently depending on their genetic makeup. This may bias the estimates of heritability and environmental influences.
- The study used a clinical diagnosis of autism, which may not capture the full spectrum of autism symptoms and severity. The diagnosis was also based on registry data, which may not reflect the current diagnostic criteria or the actual clinical assessment of the individuals. This may affect the validity and reliability of the diagnosis and the generalizability of the findings.
- The study used self-report measures of autistic traits, which may not reflect the objective or observable behavior of the individuals. The measures were also different across different ages and scales, which may affect the comparability and consistency of the results. The measures were also limited in their coverage of the different domains and dimensions of autistic traits, such as sensory processing, motor skills, or cognitive abilities. This may limit the comprehensiveness and accuracy of the assessment of autistic traits.
Faq
What is the difference between autism and autistic traits?
Autism is a neurodevelopmental condition that affects how people communicate and interact with others. Autism is diagnosed based on a set of criteria that include persistent difficulties in social communication and social interaction, and restricted, repetitive patterns of behavior, interests, or activities. Autistic traits are aspects of behavior, cognition, or perception that are related to autism, such as social problems, thought problems, and attention problems. Autistic traits can be measured in the general population, not just in people with a clinical diagnosis of autism.
How do you measure autism and autistic traits in twins?
The researchers used two different sources of data to measure autism and autistic traits in twins. For autism, they used national health registers that record diagnoses of autism and other disorders. For autistic traits, they used self-report questionnaires that assess social problems, thought problems, and attention problems. These questionnaires are part of the Child Behavior Checklist, which is a widely used tool to measure various aspects of child and adolescent mental health.
How do you estimate the heritability of autism and autistic traits?
The researchers used twin methods to estimate the heritability of autism and autistic traits. Twin methods compare the similarity of identical twins, who share 100% of their genes, and fraternal twins, who share 50% of their genes, to estimate the relative contribution of genetic, shared environmental, and nonshared environmental factors to a trait or disorder. Genetic factors are those that are inherited from the parents, shared environmental factors are those that are common to both twins in a pair, such as family background or socioeconomic status, and nonshared environmental factors are those that are unique to each twin, such as individual experiences or random events.
How does the study compare to previous research on autism and autistic traits?
The study is one of the first to use twin methods to examine the effects of age on the heritability of autism and the etiological stability of autistic traits from childhood to adulthood. Previous studies have mostly focused on children and adolescents, and have reported similar or slightly lower estimates of autism heritability and autistic trait heritability. However, these studies have used different methods and measures, and have not assessed the stability of genetic and environmental influences over time. Therefore, the study adds to the existing literature by providing a comprehensive and longitudinal perspective on the causes of autism and autistic traits.
Source:
https://acamh.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jcpp.13949