The Multiple Deficit Model of Dyslexia
A few years ago, my wife and I drove her parents to Houston in their new van. It was a top-of-the-line Honda, spacious and comfortable, but my wife’s stepdad didn’t feel able to manage Houston traffic, so we agreed to help them out. During our stay, we encountered torrential rains. One afternoon as my wife was driving, she turned a corner, and the power steering and power brakes completely disengaged.
The car was uncontrollable. We were at least a half-hour away from our hotel, stuck in a major downpour, and the car could not be steered. It was like trying to coax a 4,000-pound cinder block to perform ballet. We were all screaming in fear—the steering and brakes failed every single time the car was in more than a couple of inches of water. And since the water was always deepest at intersections full of traffic, it was quite an exciting ride.
We found out later that the car had several problems. There were a couple of factory recall items and some other contributing factors that made the car only functional in dry weather. If any part of the undercarriage got wet, the entire electrical system shorted out for the duration of the moisture contact.
Here’s the point: it’s not uncommon for a problem or an issue to have more than one contributing cause. And dyslexia and its sibling conditions also have multiple contributing causes.
Why is this important? Because the more we understand the possible factors and their interactions that can contribute to dyslexia, dysgraphia, dyscalculia, dysphonia, and dyspraxia, the better our screening procedures will be, and the earlier intervention can begin. In fact, scientists who study these factors envision a world in which infants and toddlers who exhibit enough of these factors early in life can begin to receive interventions even before dyslexia has manifested itself.
I recently found a study by some of these research scientists that I want to share with you today. But before I do, please note the following points.
First, researchers call these multiple factors “comorbidities.” This might seem like a negative term, but it is the standard term used in science and health research for any issue that has more than one contributing cause.
And second, this study focuses on something called “the multiple deficit model.” In science, a model refers to a possible explanation of why something occurs. The multiple deficit model of dyslexia (MDM) stands in contrast with early models of dyslexia that theorized it is caused by a single event or risk factor.
Many parents could object to the word “deficit,” especially when applied to their child, and yet, again, this is a standard medical/scientific term used to describe anything that isn’t working in an expected way.
Therefore, the quotes and references taken from this study are not in any way being used to make people with dyslexia or those who love them feel bad, or feel guilty, or lower their expectations. Dyslexia offers many strengths, and they are not the focus of this study. Instead, the following material drills down into the possible environmental causes of dyslexia so as to facilitate earlier accommodation, something we can all agree is of paramount importance.
And finally, all the statistics and conclusions cited in this blog are taken from other studies listed in the bibliography of the multiple deficit model of dyslexia study linked at the end of this page.
Researchers at the University of Oxford and the University of Amsterdam set out to answer the question, “Which children develop dyslexia?” While seven earlier studies link a strong heritability factor, (34–66% of kids with one dyslexic parent develop dyslexia themselves), not all kids with that risk factor do develop dyslexia. Curious, isn’t it? So dyslexia is not influenced only by genetics. What else could be at play, then?
There’s a widely-accepted catchphrase in science that states, “genetics loads the gun, the environment pulls the trigger.” In scientific parlance, genotype and phenotype. Genotype refers to the genes passed down from one generation to the next. Phenotype refers to the way a person’s genes respond to their environment. Or more specifically, the set of observable characteristics of an individual resulting from the interaction of their genotype with the environment.
So this study seeks to unpack the connections between both parents and their children in the proposed intergenerational MDM, in which both parents confer the possibility of dyslexia via intertwined genetic and environmental pathways.
A number of previous studies point to the possibility of polygenicity, or many genetic contributors. Speech-sound deficits, deficits in math and sequencing, sensory processing challenges, movement/muscle control issues, and attention and focus challenges each appear to have multiple genetic risk factors with some overlap.
For example, there is considerable overlap between processing speed issues and dyslexia and ADD/ADHD. Additionally, the authors note, “roughly 70% of the genes associated with reading ability are generalists: they also influence other learning abilities.” It stands to reason that the more genetic challenges a person is born with, the lower their tolerance to environmental influences.
It’s also important to note that the term environmental influence is in no way an accusation of neglect or fault on the part of a parent. For example, if a parent is working three jobs to put food on the table, they will be much less likely to be able to spend large amounts of time creating a rich learning environment for their child.
As the authors state, “These etiological factors produce the behavioural symptoms of developmental disorders by influencing the development of relevant neural systems and cognitive processes. Importantly, there is no single etiological or cognitive factor that is sufficient to cause a disorder.”
The authors continue, “From the MDM it follows that children at family risk experience at least some of the etiological risk factors: they inherit genetic risk factors and might experience a less rich literacy environment. Hence, it is hypothesized that at-risk children have a higher genetic and environmental liability than children without a family history of dyslexia (labeled control children). Furthermore, the at-risk children who go on to develop dyslexia are expected to show cognitive deficits (to varying degrees) in several processes. Some of these cognitive processes are expected to be affected even before the onset of reading instruction, as a consequence of etiological risk factors and deficient neural systems.
A key prediction of the MDM for family risk studies concerns the at-risk children who do not develop dyslexia. If liability to dyslexia were discrete (as would happen if only one factor, say a gene, were involved), at-risk non-dyslexic children would not differ from controls. However, according to the MDM, liability is continuously distributed. This also follows from the fact that reading ability is influenced by many genes of small effect, producing normal distributions of phenotypes (Plomin et al., 2008, p. 33).
Consequently, the MDM predicts that at-risk children without dyslexia also inherit at least some disadvantageous gene variants from their dyslexic parents, giving them a higher liability than control children, although still lower than at-risk dyslexic children. At the behavioural and cognitive level, this should translate into mild deficits in literacy skills and some of its cognitive underpinnings.”
Whew, that was a lot of science, but the multiple deficit model of dyslexia holds promise for helping researchers flag early indicators of dyslexia.
What’s the take-away from the multiple deficit model of dyslexia?
The development of dyslexia has a lot of moving parts. In addition to multitudes of different genes contributing to dyslexia, a number of environmental factors exert a measurable influence as well.
What are the challenges? Several.
First, the more contributing factors to a condition, the harder it becomes to screen for the condition. It’s not like taking a throat swab to see if you have strep throat or not. The entire picture of a child’s family, their learning and processing styles, their home environment, their attitudes and practices around reading, math, physical activities, and more all play a role. Did the child have the opportunity to develop a preliteracy foundation?
(To learn more about building a preliteracy foundation, see my blogs What Is Preliteracy, and When Can Parents Begin to Build It? and Shared Reading and Parental Vocabulary as Preliteracy Tools.)
Second, most parents are already doing all they can to enrich their child’s lives. And what if reading aloud with your child is difficult for you? Or what if you struggle to focus? Or what if you’re overwhelmed with earning a living? This information isn’t intended to stress you out or cause you to have regrets. Instead, it’s meant to point out opportunities that are still available to provide help for your child for which you can ask for help yourself.
Perhaps you’ve heard the phrase “it takes a village to raise a child.” Do you have extended family who can help with reading aloud and other interactions? What about friends and neighbors? What community programs exist at your public library? Many online sources of free audiobooks are available to help keep great stories available to struggling readers. Here’s a link: https://bookriot.com/11-websites-find-free-audiobooks-online/
The third challenge can be a lack of information. If a parent does not understand dyslexia, that parent will have difficulty addressing the challenges of dyslexia. Did you know that there are at least 9 documentaries or films that feature dyslexia? Watching them as a family creates a framework of understanding and hope, which can otherwise feel in short supply. Here’s a link: https://www.understood.org/articles/en/9-films-that-feature-dyslexia
Don’t forget, I write this blog to be a free resource for families who are dealing with dyslexia and all its sibling conditions. Please dive into the Dyslexia Articles archives to find answers to your questions.
October is Dyslexia Awareness Month! For a thorough discussion of dyslexia, you may enjoy the second edition of my award-winning book Raising a Child with Dyslexia: What Every Parent Needs to Know, available in softcover, hardcover, eBook, and audio. In addition to facts on testing and accommodation, my book gives you the tools to provide the social and emotional support children with dyslexia require. The second edition has the same great content as the first edition but now contains a very helpful bibliography and index and an exciting new cover.
And to learn more about how every student best learns to read, you may also enjoy Failing Students or Failing Schools? A Parent’s Guide to Reading Instruction and Intervention, by reading specialist and shortlisted World Literacy Award nominee Faith Borkowsky.
Cardboard Box Adventures picture books are great for shared reading and can help parents establish a strong preliteracy foundation for their children. Check out the CBA Catalog for a full list of award-winning picture books, chapter books, and resources for parents and educators. Visit my Amazon author page for more information.
Link to the featured study about the multiple deficit model of dyslexia: