Home > Got Questions?, Science, Uncategorized > If environmental factors can lead to autism, why does only one of my twin boys have autism?

If environmental factors can lead to autism, why does only one of my twin boys have autism?

This week’s “Got Questions?” response comes from Alycia Halladay, PhD, Autism Speaks’ director of research for environmental science

Today’s question came in response to my last blog post. In it, I explained that when scientists talk about the “environmental factors” that increase the risk of a disorder, they’re referring to pretty much any influence beyond genetics.

In the case of autism, the clearest evidence of environmental influence seems to surround very early events such as conception, pregnancy and birth. Those with the strongest link include parental age at time of conception (both mom and dad), maternal nutrition or illness during pregnancy, and certain birth complications.

The commenter’s question is a great one that scientists are actively exploring. The short answer is that inherited genes (DNA) and environmental factors seem to interact to influence whether an infant goes on to develop autism. So if the commenter’s twins are fraternal (meaning they share about half their DNA), the difference in their genetic makeup might explain why only one developed autism.

But what if the boys are identical twins–meaning they share exactly the same DNA? In this case, something beyond genes likely accounts for the different outcomes. Comparing the rates of autism among identical and fraternal twins provides clues.

In July, researchers used our Autism Genetic Resource Exchange (AGRE) to complete the largest autism twin study to date. They found a 70 percent overlap in autism among identical twins and a 35 percent overlap among fraternal twins. That overlap between fraternal twins is much higher than the estimated 19 percent overlap between different-age siblings.

These numbers tell us that it’s not always genes alone that determine whether a child develops autism. If it were, two identical twins would always share the same outcome, and the rate of a shared autism among fraternal twins would look more like that for different-age siblings. So we conclude that shared environmental influences are also at play.

Although twins share very similar pregnancy and birth environments, those environments aren’t exactly the same. For example, twins can have different positions in the womb or different placentas, and this can affect such environmental influences as blood and oxygen flow. Indeed, twins often have different birth weights, a known risk factor for autism.

It’s important to remember that “environmental” influences such as these don’t cause autism by themselves. Rather, if a child has a genetic predisposition for developing autism, these influences may further increase the risk.

Autism Speaks continues to fund and otherwise support research on both genetic and nongenetic risk factors for autism. EARLI is a network of researchers who follow mothers of children with autism beginning at the start of another pregnancy. IBIS is a study of early brain development in the younger siblings of children with autism. These studies depend on the participation and support of the autism community.  Please visit our Participate in Research page to learn more.

Importantly, these studies provide insights into the underlying biology of different types of autism. This in turn becomes a basis for developing ways to treat and possibly prevent autism. As always, the goal of the research we support is to improve the lives of all on the autism spectrum.

And thanks for the question. Please keep them coming.

  1. October 21, 2011 at 4:01 am

    i want to know if you believe it could be our city water supplies. my only child with autism was conceived while i was drinking city water.i have three boys and none with autism.the first two sons were conceived while i had well water. my last son i realized maybe a connection so i quit drinking the city water but maybe not before he was affected with dyslexia. maybe a connection in dyslexia and autism. it could be genes but no body on my husband or my family has a autistic child. the closest i can come is that i had a different father than my siblings and maybe my fathers side had the gene. i would like to have a gene test done. as it might help me understand as well as the autism researchers.

  2. Katie Wright
    October 21, 2011 at 9:32 am

    I have to disagree that “clearest environmental evidence surrounds conception and early events in the pregnancy.” Such a statement is more reflective of the fact prenatal environmental causes of autism have been studied approximately 10- 1 over postnatal environmental.

    Most families w/ strong interests in environmental factors contributing to autism want to see more POSTNATAL research and few believe that environmental factors are confined to the period of pregnancy. This isn’t idle conjecture but a belief based on the fact that at least 50% of toddlers regress in to autism. Something is happening to these babies AFTER they are born in many cases.

    Please stop over-stating the case of parental age, it is very unhelpful and the last thing AS should be doing to placing more blame upon parents. Irva Hertz Pinchero has studied this issue for many years and found that only 3-4% of the ASD increase could be attributed to advanced parental age. Infections during pregnancy are certainly important factors worth studying but haven’t women been getting sick during pregnancy long, long before we saw this surge in autism? It is obviously plays a role, but nothing new.

    More worthwhile studying would be the weight of newborns at birth and associated APGAR scores and vaccination histories. Then study their respective feeding issues, colic, reflux, fevers? Then tell us about the differences found between male sibs w/ autism and without.

    • Noreen
      October 21, 2011 at 2:26 pm

      Love your response. I too want to know what is turning on the babies brain/possible immune system in the brain to NOTABLE see a difference in the grown of their skull. Do tell please!

    • Sarah
      October 22, 2011 at 10:58 am

      I completely agree. Conversations about environmental factors and autism should always include post natal toxic exposures not just pre-natal. Many of our kids regressed well into their toddlerhood which strongly suggests the exposure happened post natally. Autism clusters, like the Somali autism cluster in Minneapolis is also a strong indicator that a toxic environmental exposure plays a huge role in the number of autism cases. I beleive autism is primarily an environmental health issue more so than a genetic one.

    • Catherine
      October 28, 2011 at 8:10 am

      Love you Katie for expressing things so clearly and for setting the record straight!

  3. Sarah
    October 21, 2011 at 11:27 am

    I simply love the twin studies (pointing to interaction of environment and genes). Simply, the assertion that this disease is 100% genetic is a crock.

    Forget our ASD community for a moment, spread out to include asthma, allergies, diabetes, cancer (all skyrocketing)… What curbs to pollution are being put in to place? Food safety? Water safety? Air safety? Product safety? Work place safety? Field run-off safety? Etc, etc., etc. :(

    Hurry up EARLI study! :)

  4. Jennifer Pearce
    October 21, 2011 at 11:47 am

    I have 2 autistic children and 1 that doesn’t seem to have anything going on besides a big mouth, All 3 were conceived in different states, I drank well and city water with all of them, and each environment was different, the only thing that was the same with each of my pregnancies was I moved. So, could that be a factor? I know there are so many variables that it’s hard to tell which is which.

    • Noreen
      October 21, 2011 at 2:15 pm

      Interestingly enough, they are looking at stress and cortisol(sp?) too. We also have to examine if those closest to 9/11 (New Jersey/New York) have higher incidents. Stress is definitely an important factor and should not be “factored out” of the environmental studies. I also did a move. In addition I stated to a study I believed I was still dealing with PTSD because I worked in 2 WTC on 9/11 and still have “anxiety” levels that were higher than normal. I lost a cousin who worked 10 floors above me and was also feeling Survivor Guilt. These are not “normal”, so I would not discount STRESS either. Good point!

      • Catherine
        October 28, 2011 at 8:09 am

        Noreen,I have also thought about stress after 9/11 since I was on bedrest w/#2 (PDD-NOS) and my husband was in the World Financial Center. I also clearly remember the smell. And I often wonder what toxic fumes attributed to that smell and what I may have breathed in at that time. I live about 30 miles from NYC, but I remember that smell so clearly.

  5. October 21, 2011 at 11:48 am

    Lanora, that could be a possibility in your area. However, I’ve always drank well water and my oldest child has autism while my youngest does not. I don’t think we will ever fully understand the causes. I just focus on helping him to live as normal of a life as possible.

    • October 21, 2011 at 8:13 pm

      i do believe that their is many factor from prenatal,stress,folic acid deficiency and environmental. though when i realized she had a problem my husband pulled the pipes out of the ground. the iron pipes had rusted so bad you could not put a pinky finger in it. though my daughter did not have shot at the regular intended time. but i already knew she had a problem at birth. mine could have been the cause. but why didn’t my last son born right after her did not have autism. i had more of a natural prenatal care and birth.she was born in the car at the hospital. fast births for me.they have also connected delayed births(c-sections and stopping pre-term labor) so there is no one thing as in not one autistic person is the same. my second son was born in a truck but was not born with autism. no good answers. thank you

  6. Jennifer Schwenker
    October 21, 2011 at 11:58 am

    I have identical twin boys. They both have autism.

    • Jodie Kotrlik (Washington)
      October 22, 2011 at 1:04 am

      I have 8 year old identical twin boys. Both have autism.

  7. Liz
    October 21, 2011 at 12:03 pm

    think immunizations.

    • Noreen
      October 21, 2011 at 2:17 pm

      Ditto – Especially find out the Percentage of Parents who Observed them. The researchers hear this Over and Over but are they keeping Track. I want numbers!!! I know I was a parent who participated in many studies and mentioned this to each and every one! They were NOT surprised to hear it AGAIN! Ditto!

  8. Alicia Jenkins
    October 21, 2011 at 12:05 pm

    I also have twins, and only one of them is Autistic. This has been a question of mine for quite a long time. Thank you for this article :)

  9. Jenny
    October 21, 2011 at 12:20 pm

    I have fraternal twin daughters. Ashley is fine, and Stacy is Blind and autistic. They were premature and Stacy needed a high frequency ventilator because she was having a harder time than Ashley was. As a result, Stacy suffered from Retinopathy of Prematurity and is blind. We had no experience with raising a blind child, so we were probably later than most in realizing that she was also autistic. Stacy is very talented with music. She can play anything she hears on her Keyboard, and she loves to sing. Also, she can figure out how to work any radio,cd or dvd player and loves anything that records sound. She is mostly lacking in communication skills. She says words or short phrases, but no sentences and she doesn’t answer questions, unless its repetitive. She has never officially been diagnosed with autism, because (after I asked), her pediatrician referred me to a center for autistic children and after they found out she was also blind, they referred me to a school for the blind. She had already been to that school for an evaluation before and they said she had autistic tendencies, so I left it at that. She is in the public school because the school for the blind is so far away that if she went there we would only see her on weekends. And I am not comfortable sending my child away every week when she cannot communicate to me if something bad happens to her. I went way off subject here but I wanted to share our story. My daughter is a constant source of joy in our home and I am not complaining about her at all. I just wish small towns across America were more knowledgeable about Autism!

    • Noreen
      October 21, 2011 at 2:22 pm

      Amen sister! and thanx for sharing your story! My autistic son “sang” his first sentence maybe this will happen to you. Music has healing powers and really helps to express oneself. I feel that technology will help her as well as her wonderful music abilities. She may need help with tongue muscle strength (my son did) and muscle control (he did exercises). He is now speaking. I wish your family the best. I hope someone who has similar circumstances reaches out to you (parent) and shares some things that have helped. This has been our best resource, therapists and parents. God Bless!

    • October 21, 2011 at 8:46 pm

      my daughter was not officially given the diagnosis until she was ten even though i knew at birth something was wrong. when i told them what i thought it was they questioned many of times why i wanted her label autism. that is the only way she would get some help. get the diagnosis. sorry to hear about your daughter. i can’t imagine having that along with autism. though my daughter was considered blind and deaf at nine months due to the continuous seizures for nine months. it’s amazing she is as well as she is. i am very grateful in many ways that she can be who she is today. she has showed me a lot through the pain. good luck and i do accept friend requests if i know who you are.

    • suzie
      October 22, 2011 at 5:11 am

      i would try to get her into the school for kids w autism. my 4th child has autism and the communication is key i think in overcoming alot of obsticles u deal with when dealing with an autistic child..my daughter is 8.how old are your twins??i also have 4 other children who are not autistic and they are1 boy at 23, identical twin girls who are 16 and two girls who are 8 and 7.

  10. October 21, 2011 at 12:45 pm

    its a common factor to know that there is somthing wrong with your child and find out what caused it,because we know our children and can tell somthing is wrong and slowly pin point when the closest time we felt changes started to occur whether diet,exposure to somthing,or vaccines,and although people negate this we dont get any real proof that it was not and what is inside those chemicals letter by letter to know the truth.

  11. Amna
    October 21, 2011 at 1:26 pm

    Eight years ago, i gave birth to a down- syndrome baby girl, who died after six month. After three years i had an autistic boy. Both were conceived while i was drinking spring water/ mineral water. There is no medical history of autism and down- syndrome in my and my husband’s family. I did not take any medicine during any pregnancy. My husband was epileptic, who used to take strong medicine. Can u please help me to understand, what could be the reason for my kids being like that?

    • Noreen
      October 21, 2011 at 2:09 pm

      Epilepsy and Autism are very closely related. In fact, a large group of Autistics have seizures and related disorders. It is believed that there are more than 4 different ways to develop Autism. Regressive Autism is more and more believed to come from vaccine reactions (per parents observations). This is just in the Autism community and it has been shared with parents (because Dr.’s are brainwashed that anything they do in the name of health couldn’t possibly cause a bad reaction). You have to remember also, we were vaccinated. In looking at our records, we only had about 8 shots. Yes, the LIVE polio *unsafe* and some kids the *small pox* who had reactions. The DPT has had a Long, long history of Causing Neurological Problems and that is a fact. Good luck in your quest. Again, seizures are associated but what is triggering them, if anything. I did see my son have a seizure after his shots as well as many parents hospitalized their kids immediately after vaccines, due to seizures. Seizures…up too by the way. Some important things to THINK about.

      • October 21, 2011 at 8:58 pm

        My sister has 2 children, neither one of them were vaccinated and they both have autism. I teach in a preschool special classroom. I have a family whose daughter was vaccinated and has autism, so they did not vaccinate their son, he too has autism. Many parents are refusing vaccines these days, where is the study on the percentage of children not vaccinated who end up with a form of autism?

      • Katie Wright
        October 21, 2011 at 11:46 pm

        yes research these issues for a change instead not parental age

  12. Noreen
    October 21, 2011 at 2:00 pm

    I have also heard of clusters (areas of high density). I also suspect water and drugs that were not or can not be properly removed from the water. In my old neighborhood, one block alone, we are finding that five kids who grew up there have children on the Spectrum. Most definitely, I have not seen any studies on “clusters” yet I KNOW they exist! In addition, how do the drugs in our water affect us and our offspring. As for identical twins and the difference, do any of the parents NOTE or OBSERVE Vaccine Reactions? or did the parent get drugs or vaccines while pregnant?? This is important information as well all ALLERGIES. What medical differences are they noticing, immune dsyfunction or autoimmunity possibly turned on by vaccines? As we know, every Immune System is different and reacts randomly. Each one of the Mom’s I spoke with that have a child on the spectrum did. I received a flu shot that I REGRET and had an allergic skin reaction to in addition to mercury for me and my tiny developing offspring. By the way, this is the only child who developed Autism. No flu shot for me = no Autism. Can you tell me why DOCTORS are recommending flu shots to Pregnant women and not giving us Full Disclosure? Apparently, it says on the insert they do NOT know HOW this does the fetus. Why wasn’t I TOLD THIS? My son had HUGE inflammation after his Flu shot! Is this just coincidence? In hindsight, people tend to see the reactions and when they go back, note exactly when things started happening (around vaccines 6-15 months). My baby was wheezing after his Hep B, which I recorded but was never told that there are NO STUDIES that support that a babies immune system is even developed enough to react!!!! Why such abuse of little babies?? by the Government??? Is this a form of birth control?? Disgusting! In my case I was observing closely and calling in reactions to Dr.’s (who ignored me by the way). At one “stupid” point pro-vaccine. Now, like the thousands of other parents who are suspecting that his behaviors and medical condition is reaction and medical consequence of greedy vaccine companies that don’t disclose NO EFFECTIVENESS and PROBLEMS (triple shots in particular) and take NO STEPS to correct the problem (because Our GOVERNMENT lets them off the hook completely) our kids are suffering medically and struggling every day. If you are a parent, listen up. If it was FULLY DISCLOSED to me that a they had NO IDEA what a FLU shot would do to my baby or Fully Disclosed that they can’t even PROVE that the HEP B is effective so young, I would not have Vaccinated. They are LYING to you but not Disclosing these important facts. Look closely, now they are finally admitting there is a HUGE increase, now they are FINALLY admitting, It isn’t all genetics : ) Soon, they will be admitting what the Parents already Know. Hopefully, we can start Preventing this from Effecting 30-60% of the children it is.

  13. Jenna
    October 21, 2011 at 3:56 pm

    I don’t have twins. But I have two kids. One with autism and the other without. The only difference in my pregnancies is that with my first, I was taking hormonal birth control. I became pregnant while taking the pill and I didn’t know I was pregnant until I was about 7 weeks. Given the fact that you can take 3 pills and induce an abortion, I know these pills caused damaged to my son. I was sure to be clear of birth control when conceiving baby number 2, not that I knew then what I know now.

  14. erika Salinas
    October 21, 2011 at 4:07 pm

    I have 3 year old twins my girl is very good and is just strait out smart my son has not been diagnosed with autism they told me he had pdd wish is in the spectrum but I notice a change in him when they got there vaccination at 12 months he changed because everything was going good. I think they should do more research on shots than us as parents take these risk. and its like if they dont get these shots they cant go to school so its been a journey for me I love my babies but it breaks my heart that he still cannot communicate only a few words. Some times I think …….what did I do Wrong :(

  15. Georgie Melot
    October 21, 2011 at 4:43 pm

    I would love to know about any studies that look into the effects of infant/toddlers diet on autism. As a society we have become increasingly dependent on processed food. Even the ‘healthy’ food we prepare often has preservatives. I can see a corelation to the increase in cases and the increase of processed food in the American diet.

    I am a special education teacher and have spent the past few years working with children 3-5 years old. From this experience, I have learned that my youngest son, now 25 years old had MANY autistic characteristics from birth until 3 or 4 years old. So many that I am not sure why he isn’t autistic. The only thing I can think is that he would not eat any foods that did not have a ‘custard’ texture and so I made all his baby food from fresh or frozen fruits and vegetables. He had little to no processed food in his first 12-18 months of life.

  16. lauren
    October 21, 2011 at 10:01 pm

    IT was great reading everyone stories. It made me feel like i am not alone. I have twin boys that are 6 with pdd-nos. My younger brother also has autism.

  17. Marilyn Glenn
    October 22, 2011 at 12:30 am

    I have identical twin boys, and only one has autism. They were full term, but the one with autism was one pound smaller than the other. Neither child had any noticeable issues with their vaccines. I wish there were some clear cut answers.

  18. October 23, 2011 at 9:05 pm

    it has been a great forum here for me to reach out to others. as my daughter is an adult and many of you are just learning about autism in your families. if we can educate others then we are helping our future generations. i would love to see if there are any studies as to if there is mercury or lead in the bodies of our autistic children or the older generations of adults right now with it. if this is in the body and we are not detoxifying their systems that this carries on in the body forever. which in turn they will not get better in some cases.

  19. Paula
    October 24, 2011 at 12:12 pm

    Is it possible for environmental exposure later in life to trigger Aspergers? For instance, if a child is classified as developmental delay at age 5 and then later at age 8 or 9, he shows signs of Aspergers, is it something just latent in his DNA or could it be triggered by an environmental factor such as poor air quality or something.

    • December 27, 2011 at 8:08 pm

      i do believe in environmental exposure is more of a sure thing. whether it is lead poisoning, chemicals in the water, or mercury in the fish we eat. i don’t believe it’s genetics but in rare situations. i also believe that it might be a folic acid deficiency. i hope they can find some reasons soon.

  20. Sarah
    October 24, 2011 at 12:30 pm

    Has anyone considered that we may be dealing with a mass poisoning here?

    Look at the numbers…the CDC reports that 1 in 70 boys in the US has autism. There’s been a 600% increase in the last 20 years, autism clusters have been reported in Minnesota and California.

    Graph of Autism prevalence 1975-2009:


    Here’s an excerpt for a letter written by from Isaac Pessah, Director, UC Davis Center for Children’s Environmental Health and Disease Prevention Professor of Toxicology to the U.S. Senate Committee on Environment and Public Works

    “The vast majority of public and private resources has, and continues, to support work on identifying genetic impairments associated with autism risk. From these studies we have learned that genetics alone cannot predict the majority of autism cases, the patterns of impairments, severity, nor can they predict success for current treatment modalities. Moreover, we have learned that many of the molecular and cellular systems that are associated with autism are the very same ones that are the target of environmental chemicals currently of concern to human health because of their widespread use.”


    “We know that autism prevalence continues to increase dramatically clearly implicating environmental factors in autism risk. We must identify which environmental exposures and combination of exposures are contributing to increased overall risk in the population and identify the most susceptible groups. Only by bringing together the concerted effort of multidisciplinary teams of scientists can we identify which of the >80,000 commercially important chemicals currently in production promote developmental neurotoxicity consistent with the immunological and neurological impairments identified in individuals with idiopathic autism. It is clear that their is a critical need to identify which chemicals in the environment that influence the same biological pathways known to be affected in autism. Limiting exposure to these chemicals is the only way to mitigate or prevent autism in susceptible individuals”


    • December 27, 2011 at 8:14 pm

      my husband and i have asked the same questions. food with chemicals. genetic breeding of foods. what about the food that comes from other countries? i’d like to know as i believe it’s enviromental

  21. Linda
    October 26, 2011 at 3:54 am

    I have 7 Year old Triplet Boys, only one has Autism, There were clear signs that something was not 100% right from the beginning, he cried constantly, nothing would soothe him, he hated being held or cuddled, he never looked up from his cot when I walked in the room & there was no eye contact even as a baby. All his milestones were delayed, we were doing tests from about 4 months. Only diagnosed at 3 years with Autism. He was the last born so maybe the lack of oxygen could have been anonset, he weighed the biggest out of the three.

    • December 27, 2011 at 8:29 pm

      i know they are looking at oxygen deprevation. i could also think this is a possibility. your son sounds exactly how my daughter was as a newborn. does he have seizures? sorry to here about your son. i do know what it is like 24/7 of crying from discomfort and never seemed full when i done with her bottles. i later realized she drinks excessively when she was having constipation. how old is your triplets now? good luck and hope i hope he is doing good.

  22. October 29, 2011 at 9:31 am

    My little brother was diagnosed with autism when I was only 3. Now, I’m 11 and, to this day, I still don’t know what autism is. I understand how it makes people different. But, can you explain it to me. please? I would appreciate this, really.

    • November 9, 2011 at 2:57 am

      autism is complex. it is a social and emotional disconnection. some are very intelligent and can show it. others are intelligent but can not show their interests or likes. some can speak while most others will not speak. some are so severe that they need someone to help them with everything they do. like feeding them because they won’t hold a spoon or use a cup because of the tactile problems. it is a learning process with each individual. for you to ask means that you care and are willing to learn. i have three sons that have helped with their sister and understand her more than most people do. good luck with you and your brother.

  23. November 8, 2011 at 10:13 pm

    My son was diagnosed three years ago with Autism/ADHD and his Kumon teacher introduced me to a natural product made from milk, after its production it is the closest to Mothers breast milk. Thirty years of research have gone into this product by Scientist and Doctors. This product has been scientifically and clinically proven. I tried it and in two months I saw improvements. Today my son shows no signs of Autism/ADHD, and he is doing remarkably well, focused on his school work..gets A&B and takes the initiative to do everything for himself, truly a turnaround from the way he used to be. If you are interested to find out more about this product, please email me at asha.persaud@gmail.com. Thank you.

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