The Causes of Autism Spectrum Disorder

"Why did my child develop autism?" is one of the very first questions almost every family asks during the diagnosis process. In this chapter we'll honestly share what science answers to this question today, and the points we don't yet know for certain.

A Strong Genetic Basis

The most solid evidence we have today points to autism being largely genetic. Twin studies show that when one identical twin is diagnosed with autism, the likelihood the other is also diagnosed is very high (around 70–90 percent in some studies), while in fraternal twins this rate is noticeably lower. This finding shows that heredity plays a strong role in how autism arises.

The genetic contribution isn't in the form of a single "autism gene," but is a complex (polygenic) structure in which hundreds of different genes and gene regions come together to increase the risk of autism. Most of these genes are ones that play a role in how neurons form connections with each other (synaptic development), in signal transmission, or in the timing of brain development. In some children, more prominent genetic changes that can lead to autism on their own can be found (for example, the mutation in the FMR1 gene that leads to Fragile X syndrome, the mutation in the MECP2 gene that leads to Rett syndrome, or the mutations in the TSC1/TSC2 genes that lead to tuberous sclerosis). These conditions will be covered in detail in Chapter 14 of the book.

The Risk of Recurrence in Siblings

In the siblings of a child diagnosed with autism, there is a noticeably increased risk of developing autism compared to the general population. Large-scale studies show the risk of autism in a next sibling to be around 10–20 percent, and that this risk rises further if there is more than one child with autism in the family. This information is important both for genetic counseling and for monitoring younger siblings early; when a sibling of a child diagnosed with autism is born, it is recommended that the baby's development be followed more closely than standard monitoring.

Tip / Practical Suggestion

If there is a history of autism, language delay, attention difficulties, or learning difficulties in your family, be sure to share this with your doctor during the assessment. Family history can be an important guide in directing the genetic evaluation. If you are having, or are about to have, a new baby after your child diagnosed with autism, talk this over in advance with your pediatric neurologist or developmental pediatrician.

Environmental Contributing Factors

Alongside genetic predisposition, it's thought that some environmental and prenatal/perinatal factors can somewhat increase the risk of autism. Among these are:

• Advanced maternal or paternal age — especially with advanced paternal age, it's thought that new (de novo) genetic changes that build up over time in sperm cells play a role.
• Certain infections during pregnancy or strong maternal immune activation — the theory that strong activation of the mother's immune system during pregnancy may affect the developing fetal brain is being researched.
• Very premature birth (prematurity) and low birth weight — especially in very premature babies, the risk of autism is increased compared to babies born at term.
• Exposure to certain medications during pregnancy — for example, valproate (an epilepsy and migraine medication) is a rare, high-evidence example known to increase the risk of autism when used during pregnancy; for this reason, the use of this medication in women of childbearing age requires special care.
• Folic acid deficiency during pregnancy — on the contrary, there is evidence that getting enough folic acid before and during pregnancy may somewhat reduce the risk of autism; this underlines the importance of prenatal vitamin supplementation.

It needs to be stated clearly here: vaccines, parenting style, screen time on its own, particular foods, or "cold" parenting have not been scientifically shown to cause autism. Most of these claims rest either on disproven single studies or on the misreading of coincidences in timing. None of these factors is an "autism cause" on its own; they should be thought of as contributing factors that can somewhat increase the risk in a child who has a genetic predisposition, meaning not every child exposed to these factors develops autism, and not every child with autism is found to have one of these factors.

Caution / When to Seek Advice

It is completely normal not to find a single, definite answer to the question "Why my child?"; in most cases it's thought that more than one genetic and environmental factor come together. This uncertainty in no way points to a parenting mistake.

The Gut Microbiota and the "Gut-Brain Axis" Theory

A research area that has become popular in recent years looks at the possible effects of the gut microbiota (the community of bacteria living in the gut) on brain development and behavior. Differences in the makeup of the gut microbiota have been described in some children with autism; but whether this difference is a cause of autism, a result of it (for example, a result of selective eating patterns), or an unrelated accompanying finding, is not yet clear. This area continues to be a subject of active research; for now, there is no strong, consistent evidence that microbiota-based treatments improve the core signs of autism.

What Changes at the Level of Brain Development?

Imaging and neuroscience studies show that in the brains of children with autism there are patterns different from typical development in the connectivity of certain regions (especially in the prefrontal cortex, the amygdala, and the networks known as the mirror neuron system, which are involved in social thinking). Some studies have also described a temporary speeding up of brain volume growth in early childhood; it's thought that this growth acceleration may affect the timing of the maturation of the social and language networks. But none of these findings is diagnostic on its own; a clinical diagnosis still rests on behavioral observation and developmental assessment.

Chapter Summary

In this chapter we looked at the genetic and environmental causes of autism, why the vaccine myth is scientifically invalid, and current research areas such as the gut-brain axis. In the next chapter, we'll take a detailed look at the official classification system your doctor uses to diagnose autism, the DSM-5 criteria and severity levels.