A new study examining nine consecutive birth years in Sweden indicates that the dramatic rise in clinical diagnoses of autism spectrum disorder is not accompanied by an increase in autism-related symptoms in the population. The research, published in the journal Psychiatry Research, also found that while parent-reported symptoms of ADHD remained stable in boys, there was a small but statistically significant increase in symptoms among girls.

Autism spectrum disorder, or ASD, is a neurodevelopmental condition characterized by differences in social communication and interaction, along with restricted or repetitive patterns of behavior and interests. Attention-Deficit/Hyperactivity Disorder, or ADHD, is another neurodevelopmental condition marked by persistent patterns of inattention, hyperactivity, and impulsivity that can interfere with functioning or development. Over the past two decades, the number of clinical diagnoses for both conditions has increased substantially in many Western countries, particularly among teenagers and young adults.

This trend has raised questions about whether the underlying traits associated with these conditions are becoming more common in the general population. Researchers sought to investigate this possibility by looking beyond clinical diagnoses to the level of symptoms reported by parents.

“The frequency of clinical diagnoses of ASD and ADHD has increased substantially over the past decades across the world,” said study author Olof Arvidsson, a PhD student at the Gillberg Neuropsychiatry Centre at Gothenburg University and resident physician in Child and Adolescent Psychiatry.

“The largest prevalence increase has been among teenagers and young adults. Therefore, we wanted to investigate if symptoms of ASD and ADHD in the population had increased over time in 18-year-olds. In this study we used data from a twin study in Sweden in which parents reported on symptoms of ASD and ADHD when their children turned 18 and investigated whether symptoms had increased between year 2011 to 2019.”

To conduct their analysis, the researchers utilized data from a large, ongoing project called the Child and Adolescent Twin Study in Sweden. This study follows twins born in Sweden to learn more about mental and physical health. For this specific investigation, researchers focused on information collected from the parents of nearly 10,000 twins born between 1993 and 2001. When the twins reached their 18th birthday, their parents were asked to complete a web-based questionnaire about their children’s behaviors and traits.

Parents answered a set of 12 questions designed to measure symptoms related to autism. These items correspond to the diagnostic criteria for ASD. For ADHD, parents completed a 17-item checklist covering problems associated with inattention and executive function, which are core components of ADHD.

Using this data, the researchers employed statistical methods to analyze whether the average symptom scores changed across the nine different birth years, from 1993 to 2001. They also looked at the percentage of individuals who scored in the highest percentiles, representing those with the most significant number of traits.

The analysis showed no increase in the average level of parent-reported autism symptoms among 18-year-olds across the nine-year span. This stability was observed for both boys and girls. Similarly, when the researchers examined the proportion of individuals with the highest symptom scores, defined as those in the top five percent, they found no statistically significant change over time. This suggests that the prevalence of autism-related traits in the young adult population remained constant during this period.

The results for ADHD presented a more nuanced picture. Among boys, the data indicated that parent-reported ADHD symptoms were stable. There was no significant change in either the average symptom scores or in the percentage of boys scoring in the top 10 percent. For girls, however, the study identified a small but statistically detectable increase in ADHD symptoms over the nine birth years. This trend was apparent in both the average symptom scores and in the proportion of girls who scored in the top 10 percent for ADHD traits.

Despite being statistically significant, the researchers note that the magnitude of this increase in girls was small. The year of birth explained only a very small fraction of the variation in ADHD symptom scores. The results suggest that while there may be a slight upward trend in certain ADHD symptoms among adolescent girls, it is not nearly large enough to account for the substantial increase in clinical ADHD diagnoses reported in this group. The study provides evidence that the steep rise in both autism and ADHD diagnoses is likely influenced by factors other than a simple increase in the symptoms themselves.

“Across the nine birth years examined, there was no sign of increasing symptoms of ASD in the population, despite rising diagnoses,” Arvidsson told PsyPost. “For ADHD, there was no increase among boys. However, in 18-year-old girls we saw a very small but statistically significant increase in ADHD symptoms. The increase in absolute numbers was small in relation to the increase in clinical diagnoses.”

The researchers propose several alternative explanations for the growing number of diagnoses. Increased public and professional awareness may lead more people to seek assessments. Diagnostic criteria for both conditions have also widened over the years, potentially including individuals who would not have met the threshold in the past. Another factor may be a change in perception, where certain behaviors are now seen as more impairing than they were previously. This aligns with other research indicating that parents today tend to report higher levels of dysfunction associated with the same number of symptoms compared to a decade ago.

Changes in societal demands, particularly in educational settings that place a greater emphasis on executive functioning and complex social skills, could also contribute. In some cases, a formal diagnosis may be a prerequisite for accessing academic support and resources, creating an incentive for assessment. For the slight increase in ADHD symptoms among girls, the authors suggest it could reflect better recognition of how ADHD presents in females, or perhaps an overlap with symptoms of anxiety and depression, which have also been on the rise in this demographic.

“The takeaway is that the increases in clinical diagnoses of both ASD and ADHD need to be explained by other factors than increasing symptoms in the population, such as increased awareness and increased perceived impairment related to ASD and ADHD symptoms,” Arvidsson said. “Taken together we also hope to curb any worries about a true increase in ASD or ADHD.”

The study has some limitations. The response rate for the parental questionnaires was about 41 percent. While the researchers checked for potential biases and found that their main conclusions about the trends over time were likely unaffected, a higher participation rate would strengthen the findings. Additionally, the questionnaire for ADHD primarily measured symptoms of inattention and did not include items on hyperactivity. The results, therefore, mainly speak to the inattentive aspects of ADHD.

Future research could explore these trends with different measures and in different populations. The researchers also plan to investigate trends in clinical diagnoses more closely to better understand resource allocation for healthcare systems.

“We want to better understand trends of clinical diagnoses, such as trends of incidence of diagnoses in different groups,” Arvidsson said. “With increasing clinical diagnoses of ASD and ADHD and the resulting impact on the healthcare system as well as on the affected patients, it is important to characterize these trends in order to motivate an increased allocation of resources.”

The study, “ASD and ADHD symptoms in 18-year-olds – A population-based study of twins born 1993 to 2001,” was authored by Olof Arvidsson, Isabell Brikell, Henrik Larsson, Paul Lichtenstein, Ralf Kuja-Halkola, Mats Johnson, Christopher Gillberg, and Sebastian Lundström.

A new study published in the Journal of Affective Disorders provides evidence that a brief but structured physical exercise program can help reduce stress levels in adolescents diagnosed with attention-deficit/hyperactivity disorder. The researchers found that after just three weeks of moderate to vigorous physical activity, participants reported lower levels of stress and showed a measurable increase in salivary cortisol, a hormone linked to the body’s stress response.

Adolescence is widely recognized as a time of dramatic psychological and biological development. For teens with ADHD, this period often comes with heightened emotional challenges. In addition to the typical symptoms of inattention and hyperactivity, many adolescents with the condition also struggle with internal feelings such as anxiety and depression. These emotional difficulties can interfere with daily functioning at school and at home, placing them at greater risk for long-term mental health problems.

Although stimulant medications are commonly used to manage symptoms, they often cause side effects such as sleep problems and mood shifts. Due to these complications, many families and young people stop using medication or seek alternative approaches. One such approach gaining traction is physical exercise. Prior research suggests that structured activity may benefit brain function and emotional regulation. However, most studies have focused on children rather than adolescents, and few have examined whether exercise influences cortisol, a stress hormone thought to be dysregulated in young people with ADHD.

Cortisol plays an important role in how the body manages stress. Low levels of cortisol in the morning have been found in children and adolescents with ADHD, and this pattern has been associated with fatigue, anxiety, and greater symptom severity. The researchers behind the new study wanted to know whether a short physical exercise intervention could influence both subjective stress levels and objective stress markers like cortisol in teens with ADHD.

“Adolescents with ADHD face stress-related challenges and appear to display atypical cortisol patterns, yet most exercise studies focus on younger children and rarely include biological stress markers,” explained study author Cindy Sit, a professor of sports science and physical education at The Chinese University of Hong Kong.

“We wanted to test a practical, low-risk intervention that schools and families could feasibly implement and to examine both perceived stress and a physiological marker (salivary cortisol) within a randomized controlled trial design. In short, we aimed to examine whether a brief, feasible program could help regulate stress in this under-researched group through non-pharmacological methods.”

The researchers recruited 82 adolescents, aged 12 to 17, who had been diagnosed with ADHD. Some of the participants also had a diagnosis of autism spectrum disorder, which often co-occurs with ADHD. The teens were randomly assigned to one of two groups. One group participated in a structured physical exercise program lasting three weeks. The other group served as a control and continued with their normal routines.

The exercise group attended two 90-minute sessions each week, totaling 540 minutes over the course of the program. These sessions included a variety of activities designed not only to improve physical fitness but also to engage cognitive functions such as memory, reaction time, and problem-solving. Exercises included circuit training as well as games that required strategic thinking and teamwork. Participants were guided to maintain moderate to vigorous intensity throughout much of the sessions, and their heart rates were monitored to ensure appropriate effort.

To measure outcomes, the researchers used both self-report questionnaires and biological samples. Stress, depression, and anxiety levels were assessed through a validated scale. Cortisol was measured using saliva samples collected in the afternoon before and after the intervention, as well as three months later.

The findings showed that immediately following the exercise program, participants in the exercise group reported lower levels of stress compared to their baseline scores. At the same time, their cortisol levels increased.

The increase in cortisol following exercise was interpreted not as a sign of increased stress but as a reflection of more typical hormonal activity. The researchers noted that this pattern aligns with the idea of exercise as a “positive stressor” that helps train the body to respond more effectively to real-life challenges. Importantly, the teens felt less stressed, even as their cortisol levels rose.

“The combination of lower perceived stress alongside an immediate rise in cortisol was striking,” Sit told PsyPost. “It supports the idea that exercise can feel stress-relieving while still producing a normal physiological stress response that may help calibrate the HPA axis. We also noted a baseline positive association between anxiety and cortisol in the control group only, which warrants further investigation.”

However, by the three-month follow-up, the improvements in self-reported stress had faded, and cortisol levels had returned to their initial levels. There were no significant changes in self-reported depression or anxiety in either group at any point.

“A short, three-week exercise program (90-minute sessions twice a week at moderate to vigorous intensity) reduced perceived stress in adolescents with ADHD immediately after the program,” Sit said. “Cortisol levels increased right after the intervention, consistent with a healthy, short-term activation of the stress system during exertion (often called ‘good stress’). The positive effects on perceived stress did not last for three months without continued physical exercise, and we did not observe short-term changes in depression or anxiety. This suggests that ongoing participation is necessary to sustain these benefits.”

Although the results suggest benefits from the short-term exercise program, there are some limitations to consider. Most of the participants were male, and this gender imbalance could affect how the findings apply to a broader group of adolescents. The study also relied on self-report questionnaires to assess stress, anxiety, and depression, which can be affected by personal bias. Additionally, there was no “active” control group, meaning the control participants were not given an alternate activity that involved social interaction or structure, which might have helped isolate the effects of the exercise itself.

Future studies might benefit from longer intervention periods to examine whether extended participation can produce lasting changes. Collecting saliva samples multiple times during the day could also help map out how cortisol behaves in response to both daily routines and interventions. Incorporating interviews or observer-based assessments could provide a more complete understanding of emotional changes, especially in teens who have difficulty expressing their feelings through questionnaires.

“Our team is currently conducting a large randomized controlled trial testing physical‑activity interventions for people with intellectual disability, with co‑primary outcomes of mood and physical strength,” Sit explained. “The broader aim is to develop scalable, low‑cost programs that can be implemented in schools, day services, and community settings. Ultimately, we aim to increase access for underserved populations so that structured movement becomes a feasible part of everyday care and improves their quality of life.”

“We see exercise as a useful adjunct, not a replacement, for standard ADHD care,” she added. “In practice, that involves incorporating structured movement alongside evidence-based treatments (e.g., medication, psychoeducation, behavioural supports) and working with families, schools, and healthcare providers. Exercise is accessible and generally has low risk; it can assist with stress regulation, sleep, attention, and fitness. However, it should be individualized and monitored, especially for individuals with special needs like ADHD, to support rather than replace routine care.”

The study, “Efficacy of a short-term physical exercise intervention on stress biomarkers and mental health in adolescents with ADHD: A randomized controlled trial,” was authored by Sima Dastamooz, Stephen H.S. Wong, Yijian Yang, Kelly Arbour-Nicitopoulos, Rainbow T.H. Ho, Jason C.S. Yam, Clement C.Y. Tham, Liu Chang, and Cindy H.P. Sit.

A new study challenges a leading explanation for why auditory stimulation, such as pink noise, can improve cognitive performance in people with traits of attention deficit hyperactivity disorder. The research found that both random noise and a non-random pure tone had similar effects on a brain activity measure linked to neural noise, which contradicts key assumptions of the prominent moderate brain arousal model. These findings were published in the Journal of Attention Disorders.

For years, scientists have observed that listening to random auditory noise, like white or pink noise, can benefit cognitive functioning in individuals with ADHD or elevated traits of the condition. The moderate brain arousal model was proposed to explain this phenomenon. This model is built on two primary assumptions. First, it suggests that ADHD is associated with lower-than-optimal levels of internal neural noise.

Second, it proposes that external random noise boosts this internal neural noise through a mechanism called stochastic resonance, improving the brain’s ability to process signals. However, these foundational ideas had not been sufficiently tested, particularly because most studies lacked a direct measure of neural noise or a proper non-random sound condition to isolate the effects of stochastic resonance.

Joske Rijmen and her colleagues at Ghent University aimed to directly test these two core assumptions of the moderate brain arousal model. They designed an experiment to measure neural noise directly while participants listened to different types of sound. The researchers wanted to see if ADHD traits were indeed linked to lower neural noise at baseline. They also sought to determine if the effects of sound on brain activity were specific to random noise, as the theory of stochastic resonance would predict.

To conduct their investigation, the researchers recruited 69 neurotypical adults. Participants first completed the Adult ADHD Self-Report Scale, a questionnaire used to assess the number and frequency of symptoms associated with the condition. This allowed the scientists to examine ADHD as a spectrum of traits rather than a simple diagnostic category.

Each participant then underwent a resting-state electroencephalogram, a non-invasive procedure that records the brain’s electrical activity. While their brain activity was monitored, participants sat with their eyes closed for three distinct two-minute periods: one in silence, one while listening to continuous pink noise (a random signal), and one while listening to a continuous 100 Hz pure tone (a non-random signal).

The research team analyzed the electroencephalogram data by focusing on a specific feature known as the aperiodic slope of the power spectral density. This measure reflects background brain activity that is not part of rhythmic brain waves and is considered a direct index of neural noise. A steeper slope in this measurement corresponds to less neural noise, while a flatter slope indicates more neural noise. By examining how this slope changed across the different sound conditions and in relation to participants’ ADHD traits, the scientists could test the predictions of the moderate brain arousal model.

The study’s findings presented a direct challenge to the model’s first assumption. During the silent condition, the researchers found a relationship between ADHD traits and the aperiodic slope. Individuals who reported more traits of ADHD tended to have a flatter slope. This finding suggests that they had more background neural noise, not less. The result is the opposite of what the moderate brain arousal model predicted and aligns with other recent studies that have also found evidence for increased neural noise in older children and adolescents with ADHD.

The results also contradicted the model’s second assumption regarding the mechanism of stochastic resonance. When participants with elevated ADHD traits listened to pink noise, their aperiodic slope became steeper. This change signifies a reduction in their neural noise. This outcome is contrary to the model’s suggestion that random noise should increase neural noise in this group.

Most significantly, the researchers found that the non-random pure tone had a virtually identical effect on brain activity as the pink noise. Listening to the 100 Hz tone also led to a steeper aperiodic slope, or a decrease in neural noise, in participants with higher levels of ADHD traits. The fact that a non-random sound produced the same effect as a random sound strongly questions the idea that stochastic resonance, which requires a random signal, is the necessary mechanism behind the benefits of auditory stimulation. If stochastic resonance were the driving force, only the pink noise should have produced this effect.

The authors propose that an alternative explanation may be needed. Rather than relying on stochastic resonance, both types of sound might have a more general effect on brain arousal. This idea is more consistent with the state regulation deficit account of ADHD, which suggests that individuals with the condition have difficulty regulating their arousal levels to match situational demands.

According to this view, any form of additional stimulation, not just random noise, could help modulate arousal to a more optimal state. The researchers also noted the puzzling observation that stimulation appeared to decrease brain arousal in individuals with higher ADHD traits. They speculate this might relate to difficulties these individuals have in achieving a truly restful state, and the continuous sound may have helped them to calm or regulate their brain activity.

The study has some limitations that the authors acknowledge. The research was conducted with neurotypical adults who varied in their traits of ADHD, so the findings need to be replicated in a group of individuals with a formal clinical diagnosis. Another point is that the brain activity was measured during a resting state, not while participants were engaged in a cognitive task where the benefits of noise are typically observed.

Future research should explore whether these same brain activity patterns occur during tasks that require attention and focus. Investigating these effects in a clinical sample of people with diagnosed ADHD will be an important next step to confirm these conclusions.

The study, “Pink Noise and a Pure Tone Both Reduce 1/f Neural Noise in Adults With Elevated ADHD Traits: A Critical Appraisal of the Moderate Brain Arousal Model,” was authored by Joske Rijmen, Mehdi Senoussi, and Jan R. Wiersema.