What Are the Causes of I/DD?

Intellectual and developmental disabilities, often called I/DD, can have many different causes. Some studies have linked them to genetics or changes in the way the brain develops during birth. Others relate them to complications during pregnancy or delivery, an illness or injury suffered during childhood, or even exposure to chemicals or other substances in the environment. 

In many cases, the cause is never fully identified or understood, but that is okay. What matters most is that people with I/DD receive the right care, support, and opportunities in life. 

At-a-glance:

• I/DD can be caused by genetic conditions, prenatal factors, birth complications, illness, injury, or toxic exposure.
• Some causes begin before birth, while others happen during birth or early childhood.
• Not every case has a clearly known cause, and that does not change a person's value or potential.
• Understanding the cause may help guide care, support, and treatment planning.

What Is I/DD?

I/DD stands for intellectual and developmental disabilities. These are lifelong conditions that affect the way a person learns, experiences emotions, reasons, communicates, solves problems, adapts, and lives daily. Intellectual and developmental disabilities typically start before adulthood, more commonly during birth or the developmental years, but look different in every individual.

For example, some people with I/DD live very independent lives and need minimal support. Others need more hands-on care throughout their lives. The goal of every support system is to meet each person where they are and help them live a meaningful connected life. 

What is the Difference Between an Intellectual Disability and a Developmental Disability?

These terms are related, but they are not identical. Many families, providers, and organizations use I/DD as a broader umbrella term that covers both.

Intellectual Disability (ID)

• An individual who is limited in their intellectual functioning, meaning the way they reason, learn, and solve problems.
• Individuals with intellectual disabilities struggle with adaptive behaviors, meaning daily living, communication with others, and their social skills and interactions.
• They are usually identified through IQ testing and developmental assessments.
• Traditionally defined as beginning before age 18, but some sources cap the age at 22.

Acquired brain injuries or dementia can have very similar characteristics to intellectual disabilities, but the onset of those is specifically in adulthood. ID has an early onset. 

Developmental Disability (DD)

• A broader category that includes a range of conditions that last a lifetime.
• Developmental disabilities affect the way an individual develops physically, the way they learn, their language, and the way they behave. 
• Begin before age 22, or before the developmental period ends.
• Includes conditions such as Autism Spectrum Disorder, Cerebral Palsy, and Epilepsy.

Key difference: Intellectual disability is a type of developmental disability, but not every developmental disability affects intellect. DD focuses on development. ID focuses on cognitive abilities and adaptive skills.

Read more about the differences between intellectual and developmental disabilities.

What are the Main Causes of I/DD?

Researchers and clinicians have identified factors that contribute to causing I/DD and they’ve categorized them. Some individuals may be affected by one cause, more than one, or none (sometimes we don’t know why they happen). Let’s explore them below:

Genetic Conditions 

Genetic factors are one of the most common causes of I/DD. They involve differences in an individual’s genetic makeup, meaning their chromosomes, single genes, spontaneous genetic changes, or conditions that are inherited. Some of these are:

• Chromosomal differences, like Down syndrome, which involves an extra copy of chromosome 21.
• Single-gene conditions, such as Fragile X syndrome, one of the most common inherited causes of intellectual disability.
• Metabolic and neurological conditions, such as phenylketonuria (PKU) or Rett syndrome.
• Inherited or spontaneous genetic changes, which can happen even when there is no family history.

A genetic cause does not mean two people will have the same strengths, challenges, or support needs. Every person is unique, and a diagnosis is only one piece of who they are. 

Causes During Pregnancy

Some causes of I/DD can begin before a baby is born, while the brain and body are still developing. During pregnancy, certain things may affect how the baby’s brain forms and works. These can include:

• Infections during pregnancy, like cytomegalovirus (CMV), rubella, or Zika virus.
• Exposure to alcohol, certain drugs, or other toxins.
• Differences in how the baby’s brain or body develops
• Health conditions during pregnancy, such as untreated thyroid problems or uncontrolled diabetes

Many of these causes are not within a parent’s control. Learning about them is not about blaming anyone, but about understanding what may affect development and making sure children and families get the right care and support.

Complications During Birth

Certain problems during labor or delivery can affect a baby’s oxygen levels or brain development. Many babies who have birth complications still grow and develop normally. But for some, these complications can lead to challenges that last longer or affect development. Examples may include:

• Lack of oxygen during delivery
• Premature birth (being born early), especially before 32 weeks.
• Having a very low birth weight
• Other serious problems during labor and delivery.

Illness, Injury, or Infection in Early Childhood

Some causes of I/DD can happen after a baby is born, especially during infancy and early childhood. This is a time when the brain is still growing and developing quickly. Things that may affect brain development during this time include:

• Brain injuries, such as from falls, accidents, or abuse
• Serious infections that affect the brain or nervous system, like meningitis or encephalitis
• Childhood illnesses like measles or whooping cough, especially if they are not treated or prevented
• Other health problems that can affect the brain, like strokes or seizures

Not every illness or injury leads to I/DD. But when serious health concerns happen during early childhood, it is important to get medical care and support as early as possible.

Environmental Exposures and Toxins

Environmental factors can also affect a child’s development, especially during pregnancy, infancy, and early childhood, when the brain is growing quickly. These may include:

• Exposure to harmful substances like lead or mercury
• Severe lack of nutrition during pregnancy or early childhood
• Limited access to medical care
• Exposure to certain chemicals or pollution

When the Cause Is Unknown

For many individuals, no single clear cause of I/DD can be identified, even after being tested. This is more common than many families realize.

• Sometimes testing does not lead to one clear answer.
• Not having a known cause does not change a person's value, potential, or the support they deserve.
• Care should always focus on each person's strengths, needs, and quality of life.

What Risk Factors Can Increase the Chance of I/DD?

A risk factor is something that may increase the likelihood of I/DD, but it does not mean I/DD will happen. Common risk factors include:

• Family history of certain genetic or developmental conditions.
• The age of the mother particularly very young or older individuals.
• The age of the father (there is emerging evidence that the health status of the father at conception may play a bigger role than previously thought).
• Prematurity and low weight at birth
• Health status of the mother (such as conditions like untreated diabetes)
• Lack of prenatal care or access to consistent prenatal care.

Identifying risk factors is helpful for planning, but it is not a prediction, and it’s important to remember that no one is at fault for the development of intellectual and developmental disabilities.

Can Intellectual and Developmental Disabilities Be Prevented?

Some causes of I/DD may be preventable, but many are not. 

Talking about prevention is not about blaming parents, families, or caregivers, but about understanding what can support a child’s health before birth, after birth, and during early development.

Some steps that may help lower risk include:

• Getting regular prenatal care
• Managing health conditions during pregnancy like diabetes or high blood pressure
• Avoiding alcohol, drugs, and harmful chemicals during pregnancy
• Staying up to date on recommended vaccines and taking steps to prevent infections
• Helping prevent injuries by using car seats, following safe sleep practices, and making the home safer.
• Getting pediatric care early if developmental concerns appear

Why blame is not helpful: It is important to remember that many causes of I/DD happen without warning. Sometimes there is no known risk factor, and no one did anything wrong. The focus should be on care, support, and understanding, not guilt. Every family and every person with I/DD deserves compassion and respect.

Signs That a Child May Need an Evaluation for I/DD

Every child grows and learns at their own pace. But sometimes, certain signs may mean it is a good idea to talk with a healthcare provider about an evaluation. You may want to ask about an evaluation if your child is:

• Having trouble reaching milestones, such as sitting up, walking, or talking
• Struggling with speech, learning, or social skills, such as using words, making eye contact, or playing with others
• Having a hard time with everyday tasks, such as eating, getting dressed, or following simple directions for their age

Seeing a delay does not always mean a child has an intellectual or developmental disability. It simply means it may be helpful to ask questions, get support, and better understand your child’s needs.

When to Talk With a Doctor, Specialist, or Care Team

Parents, caregivers, and family members should not wait if they feel something is different about a child’s development. Talking to someone early can help a child get support sooner, and early support can make a big difference. It may be time to talk with a pediatrician, family doctor, or developmental specialist if:

• A child is often missing milestones for their age
• Speech, social skills, movement, or coordination seem delayed or are getting worse
• You have questions about the child’s behavior, learning, or everyday skills
• A teacher, caregiver, or family member has shared concerns

You know your child and loved ones best. Trusting your instincts and asking about an evaluation can be an important first step toward getting answers and support.

How are Causes of I/DD Diagnosed?

Finding the cause of I/DD usually takes more than one appointment or test. A care team may look at a person’s health history, development, behavior, learning, and daily skills to better understand what may be going on.

Medical & Family History

Doctors often start by asking questions about pregnancy, birth, early childhood, and family health history. This can help them look for clues about what may have affected development.

Developmental Screening & Daily Skills

A provider may use screening tools or tests to understand how a person learns, communicates, moves, plays, and handles everyday tasks. This helps show how I/DD may be affecting daily life.

Genetic Testing Or Brain & Nerve Evaluations

Sometimes, doctors may recommend genetic testing, brain imaging, or a visit with a neurologist. These tests can help identify whether a genetic condition, brain difference, or nervous system issue may be involved.

Why Diagnosis Can Take Time

Finding a cause is not always simple. Some conditions have similar signs, test results may not give a clear answer, and families may need to meet with more than one specialist.

Frequently Asked Questions

What Is The Leading Cause Of Intellectual Disabilities?

Genetic conditions are some of the most common causes of intellectual disabilities. Down syndrome and Fragile X syndrome are often named as leading genetic causes. Other causes can include things that happen during pregnancy, birth complications, or serious health issues in early childhood.

Which Cause Of Intellectual Disability Is 100% Preventable?

Fetal Alcohol Spectrum Disorder, also called FASD, is considered one of the most preventable causes of intellectual disability. It is caused by alcohol exposure during pregnancy. Avoiding alcohol during pregnancy can prevent this cause.

Can I/DD Be Caused By Genetics?

Yes. Some cases of I/DD are caused by genetic differences. These differences may be passed down from a parent, or they may happen on their own while a baby is developing.

What Are The Two Most Common Genetic Causes Of Intellectual Disabilities?

Down syndrome and Fragile X syndrome are two of the most commonly known genetic causes of intellectual disability.

Does Knowing The Cause Change What Support Someone Needs?

Sometimes. Knowing the cause can help doctors recommend the right care, therapies, or services. But it does not change a person’s value, identity, or right to kindness, respect, and support that fits their needs.