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The Double-Deficit Hypothesis for Decoding Fluency

Sebastian Wren, Ph.D.

In my line of work, I have to travel quite a bit, so you can imagine how comforting it is to me when I'm told that most of the jets that I fly on have the ability to continue to fly even if one engine stops working.  I have to say, this is remarkable to me because most of the jets that I fly only have two engines to start with.  The idea that the jet could continue to work with half of the engines disabled impresses me tremendously.  Of course the jet cannot fly as efficiently or easily when it loses an engine, but it can fly and -- of much greater importance to me -- it can land.

I bring this up because it is useful to think about a jet with two engines when you think about decoding skills.   Research suggests that for most children, there are two engines that drive the development of fluent decoding skills (Lovett, Steinbach, and Frijters, 2000). For children to develop good decoding skills, ideally they should have good phonological processing skills (Engine 1), and they should also be able to process and identify visual information very rapidly (Engine 2), a process called Rapid Automatic Naming (Torgesen, Wagner, Rashotte, Burgess, and Hecht, 1997).

Engine 1 -- Phonological Processing Skills

Phonological processing skills have to do with the child's ability to identify and manipulate sounds within speech.  Typically, the assessment of phonological processing skills that is used is a test of phoneme awareness which measures the child's ability to segment and manipulate phonemes in speech.  For example, a child may be asked to remove the "LLL" sound in the word "PLAY" ("PAY") or add a "SSS" sound to the beginning of the word "TICK" ("STICK").

The reason this assessment is so important has to do with the fact that so many children (possibly as many as 40%) do not have good phonological processing skills when they enter school.  Some children, for a variety of reasons, do not develop an appreciation of what makes words rhyme, and they have little appreciation for alliteration (words that begin with the same sounds, such as "Ten Talking Turtles on the Telephone").  The sounds in speech are not important to these children -- they do not need to know that the word "SANDWICH" begins with the same sound as "SNOW" to be able to ask for a sandwich for lunch.  Phoneme awareness and indeed phonological processing skills in general are not important for oral communication, so young children often pay little or no attention to the sounds in speech before they come to school.  However, they are critically important for reading and writing (Wagner, Torgesen, and Rashotte, 1994; Gottardo, Stanovich, and Siegel, 1996; Shankweiler, Crain, Katz, Fowler, Liberman, Brady, Thornton, Lundquist, Dreyer, Fletcher, Stuebing, Shaywitz, and Shaywitz, 1995)

Engine 2 -- Rapid Automatic Naming (RAN)

The other engine I mentioned earlier -- translating visual information into a phonological code quickly and easily -- is relatively new in the field of reading research.  This skill is often described as Rapid Automatic Naming (RAN), and reading researchers do not know very much about it yet.  What we do know is that, just as some children are taller than their peers or can run faster than their peers, some children are also much faster at identifying visual information than their peers. 

If you ask a group of children to run around the block, you would expect some children to finish faster than others.  Likewise, if you were to ask a group of children to identify ten pictures of common objects as fast as they could, you would find that some children are able to identify those ten objects very quickly while others take a little more time. What is interesting is that those children who are a little slower to name pictures of objects also tend to be slower to identify letters of the alphabet or printed words from a list (Wolf and Bowers, 2000).  Without meaning to sound derogatory, some children are simply slower than other children when it comes to processing visual information.  (As I'll explain later, I am personally in the slow category.)

Being a little slower to process and identify visual information puts some students at a slight disadvantage when it comes to reading (Lovett, Steinbach, and  Frijters, 2000).  In one second of reading, a fast skilled reader is able to examine and process about five words in running text.  That translates into about 300 words read per minute by a fast skilled reader.  However a slow skilled reader may only read 230 words per minute -- about 3/4ths of the speed of a fast skilled reader.

That disadvantage is slight, but it is there none-the-less, and this difference in speed is more important when children are first learning to read than it is once they become more proficient readers. Children who naturally process visual information quickly and easily often have an easier time learning to read than their peers who tend to process visual information more slowly.  And children who process visual information more rapidly tend to get more out of the time they spend reading than their peers, and thus have an easier time developing proficient reading skills.

Double Deficit Hypothesis

These two engines, when running full throttle, help children develop decoding skills quickly and easily with only a little guidance and instruction from parents and teachers.  This is part of the reason why some children seem to learn to read very easily, while others have more trouble -- some children have good phonological processing skills and are also able to rapidly process visual information.  Other children either do not have the ability to process visual information very rapidly, or they lack phonological processing skills.  These children require a little more support from their teachers when learning to read.  Like the jet plane with only one engine, they can fly, but it may be a little bumpy at first.

But the children who need the most support from their teachers are the ones who have what is called a "double deficit" -- they come to school lacking both phonological processing skills and the ability to process visual information rapidly (Wolf and Greig Bowers (1999).  They are the students at greatest risk of reading failure.  In a sense, the cards have been stacked against them -- they are trying to learn to fly with no engines. And if their teachers do not intervene and provide a great deal of targeted support as soon as they come to school in kindergarten, they will rapidly slip behind their peers and may never learn to read proficiently.

Overcoming and Succeeding

A great deal of research has surfaced over the past 35 years focused on children who have difficulty processing phonological information and who consequently are not aware of the phonemes in speech.  We know a virtually everything we need to know now about identifying children who lack phoneme awareness and teaching children to develop phoneme awareness so they can develop proficient decoding skills. Unfortunately, less is known about Rapid Automatic Naming.  It is not as easy to identify at a young age, and even when identified, improving RAN and visual processing speed is considerably more difficult than helping children develop phoneme awareness.

However, there is still a road to success available, and good teachers can navigate their students down that road easily (Lovett, Steinbach, and  Frijters, 2000).  As I said, it is possible to learn to fly with one engine, and it is much, much easier to teach children to be aware of phonemes in speech than it is to make them process visual information faster.  Ensuring that all children develop phoneme awareness in kindergarten is a good way to ensure that all children have at least one engine to start with. From there, it may still be difficult for some children, but with continued support from their teachers, they can still get off the ground.

A Personal Story

As I mentioned earlier, I am a slow reader.  The last time I was tested objectively (when I was an undergraduate in college) the Nelson-Denny indicated that my comprehension scores were in the 99th percentile, but my reading rate was in the 3rd percentile.  I read fast enough to comprehend well, but nowhere nearly as fast as my peers.  Apparently, I learned to fly with just one engine -- because of some good instruction early in my life, I learned to read proficiently enough to make it through college and graduate school even though I read very slowly.  Exactly the same thing can be said of my brother, a national merit finalist in high school.  And my father, a geophysicist.  These things tend to run in families, and in my family, most of us are very slow readers.

Fifteen years have passed since the Nelson-Denny revealed that my reading rate is in the 3rd percentile, and I would not be at all surprised to find that my reading rate has increased over the years. Since college, especially in graduate school, I developed the habit of reading between 4 and 6 hours a day.  I have no data, but I am sure that all of that practice has increased my reading rate substantially.  It is not easy to affect Rapid Automatic Naming, but if anything is going to do it, practice has got to be the answer. Because of my reading habits, I process between sixteen million and twenty-four million words a year.  To put that in perspective, one year is equal to 31.4 million seconds, so I read about one word for every two seconds of my life.

Of course, I must confess that not all of the reading that I do every day is challenging, technical text.  The most recent book I read cover-to-cover was "Harry Potter and the Order of the Phoenix."  And while my current bedtime reading may seem more challenging (Sally Shaywitz's book, "Overcoming Dyslexia"), I assure you that for me, it is not.  I am so familiar with research literature in the area of reading acquisition that Shaywitz's book is easy reading for me.

In other words, while I read a lot more than most of my peers, most of the reading that I do is for pleasure, and most of it does not really stretch the boundaries of my reading comprehension skills. For children who are slow readers, that is the formula for success -- they must read much, much more than their faster peers, and one good way to encourage them to do that is to encourage them to spend a lot of time reading easy text for pleasure (see V is for Volume).

Speaking from personal experience, I would say that reading may come easier to other people, and other people may be able to read faster than I do, but for a slow reader who had a little difficulty getting off the ground with one good engine, nobody enjoys reading more than I do.


Gottardo, A., Stanovich, K. & Siegel, L. (1996). The relationships between phonological sensitivity, syntactic processing and verbal working memory in the reading performances of third-grade children. Journal of Experimental Child Psychology, 63: 563-582.

Lovett, M. W., Steinbach, K. A., &  Frijters, J. C. (2000). Remediating the Core Deficits of Developmental Reading Disability: A Double-Deficit Perspective. (Journal of Learning Disabilities, 33(4), 334-358.)

Shankweiler, D., Crain, S., Katz, L., Fowler, A., Liberman, A., Brady, S., Thornton, R., Lundquist, E., Dreyer, L., Fletcher, J., Stuebing, K., Shaywitz, S. & Shaywitz, B. (1995). Cognitive profiles of reading-disabled children: Comparison of language skills in phonology, morphology and syntax. Psychological Science, 6: 149-156.

Torgesen, J.K., Wagner, R.K., Rashotte, C.A., Burgess, S.R. and Hecht, S.A. (1997). The contributions of phonological awareness and rapid automatic naming ability to the growth of word reading skills in second to fifth grade children. Scientific Studies of Reading, 1, 161-185.

Wagner, R., Torgesen, J. & Rashotte, C. (1994). Development of reading-related phonological processing abilities: New evidence of bidirectional causality from a latent variable longitudinal study. Developmental Psychology, 30: 73-87.

Wolf M, & Greig Bowers P. (1999). The double-deficit hypothesis for the developmental dyslexias. Journal of Educational Psychology, 91: 415-438.

Wolf, M. & Bowers, P. (2000). The question of naming-speed deficits in developmental reading disability: An introduction to the Double-Deficit Hypothesis. Journal of Learning Disabilities, 33, p. 322-324. (Special Issue on the Double-Deficit Hypothesis; Special Issue Editors: M. Wolf & P. Bowers).

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Last Updated 1-2-05