Robert F. Kennedy Jr.’s False Claims link autism to childhood vaccinations is receives new review now that President-elect Donald Trump has chosen him to lead the Department of Health and Human Services, a sprawling agency with a budget of 1.7 trillion dollars which oversees research on both autism and vaccines.

The myth that autism is caused by childhood vaccines – proposed in 1998 by a British doctor who was later banned from practicing medicine in the UK — has been thoroughly exposed. Hundreds of studies have found vaccines to be safe. The World Health Organization estimates that immunizations over the past 50 years has saved 154 million lives around the world.

Kennedy, who advocates a number of health-related conspiracy theorieshave pointed to vaccines to explain the substantial increase in autism diagnoses in recent decades, which have increased from an estimated 1 in 150 children in 2000 to 1 in 36 today. Research suggests that much of that increase depends on increased awareness and investigation for the condition; changing definitions of autism to include milder conditions in the spectrum that were not recognized in previous years; as well as advances in diagnostic technology.

“For a very long time, the anti-vaccine movement has exploited families of autistic people, promoting a market for pseudoscientific treatments that don’t provide the answers they’re looking for and can expose autistic people to real harm, said Ari Neeman, co-founder of the nonprofit Autistic Self Advocacy Network and assistant professor of health policy and management at the Harvard TH Chan School of Public Health. “More discredited conspiracy theories linking autism and vaccines are not the answer.”

Timothy Caulfield, director of research at the University of Alberta’s Health Law Institute in Canada, which studies health misinformation, said people are often more willing to believe conspiracy theories about conditions like autism, whose causes are complex and not fully understood, than diseases with clear causes .

People seem less inclined to speculate about, for example, alternative explanations for Down syndrome, which causes intellectual disabilities and has long been known caused by an extra copy of chromosome 21.

“It’s a real shame because there are vulnerable families (of people with autism) who need our support,” said Judith Miller, a clinical psychologist and senior scientist and director of education at the Center for Autism Research at the Children’s Hospital of Philadelphia. “Every dollar and hour spent trying to debunk a conspiracy theory is a dollar and hour lost that could have been spent trying to understand how to help families.”

A complex condition

Finding the causes of autism is complicated, because it is not a single disorder, says Manish Arora, professor of environmental medicine and climate science at the Icahn School of Medicine at Mount Sinai in New York.

“Autism is a spectrum, not a single narrow disease,” Arora said. “There are many, many things under one umbrella.”

Although people diagnosed with autism often have similar strengths and challenges, “There are many pathways to autism and many presentations of autism,” Miller said.

Researchers have found a variety of risk factors for autism—most of which exists before birth — but there is no single cause for a neurological and developmental condition that affects how people interact with others, communicate, learn and behave.

A number of the characteristics sometimes seen in people with autism – such as being sensitive to loud noises, for example, or having difficulty interpreting social cues – are also found in people who have not been diagnosed with autism. Doctors diagnose autism based on a person’s behavior, noting that there is no simple test for the condition, like there is for Covid or diabetes, said Arora, founder and CEO of a startup that researching biomarkers for autism and other neurological conditions.

Finding the cause of an infectious disease – such as influenza, which is caused by the influenza virus – is much easier.

While researchers continue to study the factors that influence the development of autistic traits, “the only thing we know does not cause autism is vaccines,” said Catherine Lord, a psychologist and researcher at the Center for Autism Research and Treatment at the UCLA David Geffen School of Medicine.

Robert F. Kennedy Jr., who could play a key role on public health issues during Trump’s second administration, offered more details about what he would do in that role.

Genetic vulnerability

Doctors have long known that genes play a large role in autism, simply by noting that autism can run in families. For example, in identical twins – who share all their DNA – if one twin has autism, the other usually has it too. In the case of twins – who share about half their DNA – if one twin has autism, the chance that the other will have autism ranges from 53% to 67%, according to an analysis of research studies.

Researchers have identified more than 100 genes related to autism, Miller said, and genes are thought to play a role in 60% to 80% of cases.

“The genetics of autism have never been better understood,” says Dr. Gregory Cejas, medical director of the Autism Clinical Center and Fragile X Clinic at Washington University School of Medicine. “We are making great progress in the known genetic causes of autism.”

Yet genes clearly do not explain all cases of autism.

Autism is very different from conditions like sickle cell anemia or cystic fibrosis, which are caused by a single gene. Researchers believe that people develop autistic traits because of a combination of genetic vulnerability and environmental exposure, Lord said.

“People have found many, many different genetic patterns associated with autism, but none of them are only associated with autism and none of them are always associated with autism,” Lord said.

For example, fragile X syndrome – caused by a mutated gene on the X chromosome – is the most common known cause of autism. But only a fraction of children with the genetic mutation actually develop autism, Miller said. It is possible that this mutation makes some people more vulnerable to developing autistic traits, while others with the same mutations do not develop autistic traits, because they are protected by protective factors that have not yet been identified.

Some people blamed the measles-mumps-rubella vaccine for autism because symptoms of the condition often appear at about 12 to 15 months of age, the same time toddlers receive that immunization. But Miller notes that “most of the genetic conditions that affect our lives and health are not apparent at birth. Symptoms or characteristics will not show up until later, but the genetic code will have been with us all along.”

Here’s what you need to know about Robert F. Kennedy Jr., the controversial member of one of America’s most famous political families.

Prenatal vulnerability

Many of the known risk factors for autistic traits occur before birth or at the time of delivery, Arora said. Babies like experience complications at birthsuch as having their umbilical cord wrapped around their neck, have a higher risk of autism.

So do babies born too earlymaybe because of something that happened in the womb.

Children are also slightly more likely to be diagnosed with autism if they have older fathers and possibly if they have older mothers, Miller said. It is not clear whether something in the biology of older parents puts a child at a higher risk for autism, or whether socioeconomic issues may play a role. It is possible that older parents have better access to healthcare, making it more likely for their children to receive an autism diagnosis.

A mother’s health affects her child’s autism risk in several ways, according to several studies: Children have a greater chance of being diagnosed with autism if their mothers were exposed to high levels of air pollution or developed a serious infectionsuch as flu or pneumonia, when you are pregnant.

While Ne’eman, of the Autistic Self Advocacy Network, said he is not opposed to basic biological research into autism or its causes, he said these studies do little to help people with autism overcome the obstacles they face in their everyday lives .

He notes that only 8.4% of the $419 million spent on autism research in the United States devoted to support and services for people with autism.

“We need an autism research agenda,” he said, “that reflects the real priorities for autistic people and our families: support across the lifespan and inclusion in society.”

This article first appeared on NBCNews.com. Read more from NBC News here: