This article lists all the changes to the Autism diagnostic criteria introduced by DSM-5 in 2013:
DSM-5 and autism: Frequently asked questions
Autism Speaks (undated)
https://www.autismspeaks.org/dsm-5-and-autism-frequently-asked-questions
At the time it was anticipated that those changes might reduce rather than increase the number of people diagnosed with autism.
Why Combine Autism and Asperger’s?
Childmind - Last reviewed or updated on October 30, 2023.
"When the DSM-5 was released in 2013, Asperger’s and PDD-NOS were eliminated as separate conditions that children could be diagnosed with. The criteria for an autism diagnosis also changed.
The new criteria collapsed what were three separate categories of behaviors into two: persistent deficits in social communication and social interaction, and restricted, repetitive patterns of behavior. They also reduce the number of symptoms within each category while increasing the number required for diagnosis, effectively limiting the “menu” and, to critics, making the diagnosis more restrictive.
Others argue that any fall off in diagnosis should be very modest, or perhaps diagnoses might even increase under the DSM-5 criteria. Many experts have said the revisions are not at all intended to kick people off the spectrum, but rather fine-tune the diagnosis so it is more useful and representative of data gathered over the past 20 years."
https://childmind.org/article/why-combine-autism-and-aspergers/
There were also earlier changes that broadened the diagnostic criteria. So it really depends on what starting point people are referring to when they say that the diagnosis has broadened. This article is mostly but not entirely about the merging of Asperger's into autism and includes info about genetics that you might be interested in:
Is the Definition of Autism Too Broad?
Psychology Today - 2018
"Someone like me would not have been diagnosed with autism at all under the rules of DSM III. A few years later, with the arrival of DSM IV, I was diagnosed with Asperger’s (now a descriptor for autism) and later under DSM5, I was re-diagnosed on the autism spectrum."
https://www.psychologytoday.com/us/blog/my-life-aspergers/201806/is-the-definition-autism-too-broad
From the same article:
"Genetic studies have identified many variations that are implicated in autism. Some are stable and heritable, supporting the neurodiversity paradigm; while others are one-off mutations that suggest pathology.
Some genetic markers for autism are associated almost exclusively with very severe disability. Most are not; they can be present in a person whose IQ and other traits may be anywhere on the curve at all.
Other biomarker studies have identified differences in brain connectivity or brain plasticity between autistic and allistic people. Some of those studies show promise in separating autistic and allistic populations, but most cannot resolve where an individual might fall on either of the spectrums.
Interestingly, most biomarker research seems to classify people as autistic in line with the current, expanded, definition of the autism spectrum. That supports the idea that this broader spectrum is the more correct definition."
IMHO this point made in the same article is very pertinent to which diagnostic labels tend to be preferred at any particular time!
"Diagnostic labels tend to follow the money. If there is more funding for intellectual disability more people will be diagnosed with ID."
Others attribute the increase in autism diagnoses to a broader diagnosis plus better screening. This article describes the situation in the USA.
Is There an Autism Epidemic?
Johns Hopkins University - 2025
"There has been a nearly 300% increase over the past 20 years, but if you look at any two-year period across the sites that are monitoring the number of children identified with autism, it’s somewhere between a 10%–20% increase every two years.
There are two main reasons for the increase. The first is the broadened definition of Autism Spectrum Disorder, which means that more people are meeting this definition now than previously.
Second, there have been many widely successful public health programs that increased screening at wellness visits for children ages 18–24 months to look for signs of autism. Parents, caregivers, and community members are also more aware of the symptoms, and autism is being more accepted in the community. People are not as afraid to seek help and know where to go when they have concerns."
"There have been a few studies over the past two years that have looked at a specific subgroup of individuals on the spectrum who frequently need 24-hour-a-day support and care from a caregiver, often have very limited verbal communication skills, or have intellectual disability that co-occurs with autism.
The data has shown that rates of autism for that subgroup have increased minimally, if at all, over the past nearly 10 years.
Rates have increased the most for individuals who are not in that subgroup, who may have more subtle phenotypes and fewer significant co-occurring conditions. We think this is most likely due to our ability to identify these subtle traits and symptoms better."
https://publichealth.jhu.edu/2025/is-there-an-autism-epidemic
So the picture is not as simple as just Asperger's being incorporated into the Austism/ASD diagnosis nor that changes in diagnostic criteria alone are responsible for more children and adults receiving an ASD diagnosis.
I am long enough in the tooth to have seen changes play out in the UK since before 1980, when DSM-3 introduced "Infantile Autism".
How autism became autism: The radical transformation of a central concept of child development in Britain
The History of Human Sciences - 2013
Abstract
"This article argues that the meaning of the word ‘autism’ experienced a radical shift in the early 1960s in Britain which was contemporaneous with a growth in epidemiological and statistical studies in child psychiatry. The first part of the article explores how ‘autism’ was used as a category to describe hallucinations and unconscious fantasy life in infants through the work of significant child psychologists and psychoanalysts such as Jean Piaget, Lauretta Bender, Leo Kanner and Elwyn James Anthony. Theories of autism were then associated both with schizophrenia in adults and with psychoanalytic styles of reasoning. The closure of institutions for ‘mental defectives’ and the growth in speech therapy services in the 1960s and 1970s encouraged new models for understanding autism in infants and children. The second half of the article explores how researchers such as Victor Lotter and Michael Rutter used the category of autism to reconceptualize psychological development in infants and children via epidemiological studies. These historical changes have influenced the form and function of later research into autism and related conditions."
A couple of key points, bearing in mind that this article was published in 2013, so is covering increased diagnosis in the UK before DSM-3:
"There has been a phenomenal increase in diagnoses of autism (in Britain) since the 1960s . . . diagnostic rates of autism did not increase as much in France, where there was no great release of ‘retarded’ children from confinement in the 1960s and where children with developmental problems continue to receive institutional residential care up to the present day."
"Another reason why diagnoses of autism have risen in Britain and elsewhere is because the closure of institutions for ‘mentally retarded’ children led parents to campaign for better diagnosis and recognition of their children’s problems."
Full article, no pay wall:
https://journals.sagepub.com/doi/10.1177/0952695113484320
This history is also a good read IMHO, highlighting some missteps along the way and current controversies:
The Diagnosis of Autism: From Kanner to DSM-III to DSM-5 and Beyond
Journal of Autism and Developmental Disorders - 2021
https://link.springer.com/article/10.1007/s10803-021-04904-1