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Why Being Young for Grade Increases Odds of ADHD Diagnosis and Stimulant Medication

ADHD is the most com­monly diag­nosed neu­robe­hav­ioral dis­or­der in chil­dren and sub­stan­tial evi­dence indi­cates that bio­log­i­cal fac­tors play an impor­tant role in its devel­op­ment. For exam­ple, although the exact mech­a­nism by which genetic fac­tors con­vey increased risk for ADHD remains unclear, the impor­tance of genetic trans­mis­sion has been doc­u­mented in a num­ber of pub­lished studies.

Even though bio­log­i­cal fac­tors are widely regarded as impor­tant in the devel­op­ment of ADHD, no med­ical or bio­log­i­cal test is rec­om­mended for rou­tine use when diag­nos­ing ADHD. Instead, like vir­tu­ally all psy­chi­atric dis­or­ders, ADHD is defined by a con­stel­la­tion of behav­ioral symp­toms that are gen­er­ally reported on by a child’s par­ents and teacher. Also, in nearly all cases, it is par­ents’ and/or teach­ers’ con­cerns about a child’s abil­ity to focus and reg­u­lar their behav­ior that leads to a child being eval­u­ated for ADHD in the first place.

While some chil­dren dis­play suf­fi­cient inat­ten­tive and/or hyperactive-impulsive behav­ior to be diag­nosed with ADHD as preschool­ers, it is gen­er­ally not before chil­dren enter school that con­cerns related to atten­tion and hyper­ac­tiv­ity arise. This may be espe­cially true for inat­ten­tive symp­toms, as demands for sus­tained atten­tion become much greater when chil­dren start in school. Teach­ers can observe how a child’s abil­ity to reg­u­late atten­tion and behav­ior com­pares to an entire class­room — some­thing par­ents typ­i­cally can’t do — and their judge­ments may thus be par­tic­u­larly influ­en­tial in whether a child is eval­u­ated for ADHD and diag­nosed with the disorder.

A num­ber of fac­tors may con­tribute to dif­fer­ences in children’s abil­ity to focus and reg­u­late their behav­ior when they enter school. One fac­tor cer­tainly is ADHD, as chil­dren with the con­di­tion will be observed by teach­ers to be more inat­ten­tive and/or hyper­ac­tive. Another fac­tor — and one that may be fre­quently over­looked — is their age rel­a­tive to most of their class­mates. This is the issue inves­ti­gated in the stud­ies that are sum­ma­rized below.

Pub­lic school sys­tems have spe­cific dates that a child must be born by to begin kinder­garten. Con­sider two chil­dren in a school sys­tem where the cut-off is Decem­ber 31st. Jack is born on Decem­ber 31st, 2007 and would thus be eli­gi­ble to enter kinder­garten dur­ing fall 2012. Com­pared to most of his class­mates who were born as early as 1/1/2007, he will be rel­a­tively young. On aver­age, in fact, Jack would be about 6 months younger than his peers.

John is born on Jan­u­ary 1st 2008 and would thus be inel­i­gi­ble to enroll in the fall. Instead, he would need to wait until fall 2013 before start­ing kinder­garten. Thus, com­pared to most of his class­mates who could be born as late as 12/31/2008, he will be rel­a­tively old; on aver­age, he would be about 6 months older.

Although an age dif­fer­ence of 6 roughly may make lit­tle if any dif­fer­ence in the abil­ity of older chil­dren and ado­les­cents to focus, attend, and reg­u­late their behav­ior, it may make a sub­stan­tial dif­fer­ence in 5 and 6 year-olds. And, dif­fer­ences in nearly a year — which may be present between the old­est and youngest child in a grade — could be asso­ci­ated with large dif­fer­ences on these dimen­sions. This sug­gests that chil­dren rel­a­tively young for grade at the start of school will, on aver­age, be less able to reg­u­late their atten­tion and behav­ior than their class­mates. As a result, young-for-grade chil­dren may be more likely to be seen as strug­gling by teach­ers who would con­vey their con­cerns to par­ents. In many cases, this may lead par­ents to have their child eval­u­ated for ADHD and poten­tially increase the rate of ADHD diag­no­sis and treat­ment in young-for-grade chil­dren. Is there evi­dence that this is the case?

Three recently pub­lished stud­ies pro­vide com­pelling evi­dence that a child’s age rel­a­tive to his or her class­mates is an impor­tant fac­tor in whether they are diag­nosed for ADHD. Results from these stud­ies are sum­ma­rized below.

Study 1

The first study of this issue [Evans, et al., (2010). Mea­sur­ing inap­pro­pri­ate med­ical diag­no­sis and treat­ment in sur­vey data: The case of ADHD among school-age chil­dren. ‚i>Journal of Health Eco­nom­ics, 29, 657–693] used data from the National Health Inter­view Sur­vey (NHIS), an annual sur­vey of house­holds in the US that col­lects data on the extent of ill­ness, dis­ease, and dis­abil­ity in the civil­ian pop­u­la­tion. The infor­ma­tion col­lected includes whether sam­ple mem­bers had been diag­nosed with ADHD and pre­scribed stim­u­lant medication.

The authors used sur­vey data from 1997 to 2006 and only included chil­dren from states with a state-wide birth date cut-off for school entry in place when the child was five. Based on this cut-off, which var­ied by state, they exam­ined ADHD diag­no­sis and treat­ment rates for over 35,000 7 to 17 year olds who were born up to 120 days before (i.e., rel­a­tively young for grade) or up to 120 days after (i.e., rel­a­tively old for grade) the state cut-off.

Results indi­cated that 9.7% of young-for-grade chil­dren had been diag­nosed with ADHD com­pared to 7.6% of those rel­a­tively old-for-grade, a dif­fer­ence of approx­i­mately 27%. Rates of stim­u­lant usage were also sig­nif­i­cantly dif­fer­ent, 4.5% vs. 4%.

Study 2

A sec­ond study [Elder (2010). The impor­tance of rel­a­tive stan­dards in ADHD diag­no­sis: Evi­dence based on exact birth dates. Jour­nal of Health Eco­nom­ics, 29, 641–656] used data from another large national data set — the Early Child­hood Lon­gi­tu­di­nal Study — to exam­ine this issue. The data set ini­tially included over 18,600 kinder­garten stu­dents from over 1000 kinder­garten pro­grams in the US in the fall of 1998; chil­dren were fol­lowed peri­od­i­cally through 2007 when most were in 8th grade. Avail­able infor­ma­tion includes par­ent and teacher rat­ings of children’s ADHD symp­toms, diag­noses, and stim­u­lant med­ica­tion treat­ments; final results were based on over 11,750 children.

ADHD diag­no­sis and treat­ment rates were cal­cu­lated for chil­dren born the month before (young-for-grade) and the month after (old-for-grade) the state man­dated cut-off, which was Sep­tem­ber 1 for some states and Decem­ber 1 for oth­ers. For states with the Sep­tem­ber 1 cut-off, 10% of chil­dren born in August were diag­nosed with ADHD com­pared with 4.5% born in Sep­tem­ber. Rates of stim­u­lant med­ica­tion treat­ment were 8.3% vs. 2.5% respec­tively. For states with a Decem­ber 1st cut-off, the diag­no­sis rate for chil­dren born in Novem­ber was 6.8%, more than triple the 1.9% rate for those born in Decem­ber; rates of stim­u­lant treat­ment were 5.0% and 1.5% respectively.

The author exam­ined the impact of rel­a­tive age on whether chil­dren were diag­nosed with learn­ing prob­lems other than ADHD, includ­ing devel­op­men­tal delays, autism, dyslexia, socio-emotional behav­ior dis­or­der, or other learn­ing dis­abil­i­ties. For these other learn­ing prob­lems, no relative-age effects were found.

The author also demon­strated that school start­ing age had a much stronger effect on teach­ers’ per­cep­tions of children’s ADHD symp­toms than on par­ents’ per­cep­tions. He sug­gests this may be because teach­ers rate children’s behav­ior rel­a­tive to other chil­dren in the class, and rel­a­tively young chil­dren are less able to reg­u­late their atten­tion and behav­ior. Par­ents, in con­trast, may use more absolute stan­dards since they are less above to observe their child in rela­tion to a class­room full of peers.

Study 3

The final study [Mor­row et al., (2012). Influ­ence of rel­a­tive age on diag­no­sis and treat­ment of attention-deficit/hyper­ac­tiv­ity dis­or­der in chil­dren.Cana­dian Med­ical Asso­ci­a­tion Jour­nal, DOI:10.1503/cmaj.11619] exam­ined the asso­ci­a­tion between age-for-grade and ADHD diag­no­sis in a study of over 935,000 youth from British Colum­bia who were 6–12 years of age at any time between Decem­ber 1997 and Novem­ber 2008. Thus, the value of this study is that the sam­ple comes from a dif­fer­ent coun­try and entirely dif­fer­ent health care sys­tem than the US.

The cut-off for school entry in British Colum­bia dur­ing this time was Decem­ber 31. Sim­i­lar to the results reviewed above, boys born in Decem­ber were 30% more likely to be diag­nosed with ADHD than boys born in Jan­u­ary; girls born in Decem­ber were 70% more likely to be diag­nosed with ADHD than girls born in Jan­u­ary. Boys were 41% more likely and girls were 77% more likely to be treated with med­ica­tion if they were born in Decem­ber rather than January.

Sum­mary and Implications

Results from 3 inde­pen­dent stud­ies that employed large and rep­re­sen­ta­tive sam­ples indi­cate that chil­dren who are young for their grade are sig­nif­i­cantly more likely than peers to be diag­nosed with ADHD and to be treated with stim­u­lant med­ica­tion. Based on addi­tional analy­ses con­ducted in one of these stud­ies, the rel­a­tive age effect is pri­mar­ily related to teach­ers’ per­cep­tions and does not extend to other learn­ing dis­or­ders. These lat­ter two issues were exam­ined in only one of the three stud­ies, how­ever, and thus require replication.

Why might being young for grade increase the odds of a child’s being diag­nosed with ADHD? One plau­si­ble expla­na­tion is that focus­ing atten­tion and reg­u­lat­ing behav­ior are abil­i­ties that develop over time. At school entry, being up to 12 months younger than class­mates rep­re­sents a sub­stan­tial por­tion of a child’s total age, and these capac­i­ties have had less time to develop. As a result, rel­a­tively young chil­dren will gen­er­ally be less capa­ble than class­mates of reg­u­lat­ing their atten­tion and behav­ior and more likely to be iden­ti­fied by teach­ers as strug­gling on these dimen­sions. They will thus be referred for eval­u­a­tion and diag­nosed with ADHD at higher rates.

It is impor­tant to note that none of the researchers sug­gest that their data raise ques­tions about the valid­ity of ADHD as a ‘real’ dis­or­der with neu­ro­bi­o­log­i­cal under­pin­nings. In my view, using these find­ings to ques­tion the valid­ity of the con­di­tion would be highly problematic.

Instead, these find­ings sug­gest that many chil­dren who are young for their grade are diag­nosed not because they have the dis­or­der but because they are devel­op­men­tally less advanced than many of their class­mates. By the same token, chil­dren who are rel­a­tively old for their grade may be under­diag­nosed because their inat­ten­tive­ness and hyper­ac­tiv­ity do not seem exces­sive in rela­tion to their younger class­mates. Both out­comes are poten­tially harm­ful and speak to the com­plex­i­ties involved in diag­nos­ing ADHD but not to the valid­ity of ADHD as a legit­i­mate disorder.

Results from these stud­ies high­light the impor­tance of care­ful and accu­rate diag­nos­tic eval­u­a­tions. These stud­ies make an impor­tant con­tri­bu­tion to the field by rais­ing aware­ness of the role that rel­a­tive age can play in increas­ing or decreas­ing the risk of receiv­ing an ADHD diag­no­sis. Although there is no easy way to address this com­pli­cat­ing fac­tor, there are sev­eral steps that may be use­ful to take.

First, clin­i­cians eval­u­at­ing young chil­dren should be espe­cially care­ful when that child is also young rel­a­tive to his class­mates. For chil­dren born close to the cut-off for school entry, spe­cial con­sid­er­a­tion should be given to whether rel­a­tive age may be an impor­tant fac­tor in the child’s behav­ior at school.

Sec­ond, there may be value in nar­row­ing the age ranges used in many of the widely used behav­ior rat­ing scales. Results from these stud­ies sug­gest that there are sig­nif­i­cant nor­ma­tive dif­fer­ences in inat­ten­tive and hyper­ac­tive symp­toms between chil­dren born dur­ing dif­fer­ent months in the same year, let alone in dif­fer­ent years. What is ‘nor­mal’ for a child 6 years and 1 month old dif­fers from what is typ­i­cal for a child 6 years 11 months old.

How­ever, behav­ior rat­ing scales gen­er­ally have age cat­e­gories that encom­pass mul­ti­ple years. Thus, rather than com­par­ing whether the inat­ten­tive behav­iors a teacher reports for a young 6 year old are exces­sive rel­a­tive to other young 6 year old’s, the child’s score will be deter­mined in rela­tion to a ‘nor­ma­tive group’ that includes chil­dren who are sev­eral years older. As a result, chil­dren at the low end of the age range may be more likely to receive ele­vated ADHD symp­tom rat­ing scores than chil­dren at the upper end of the age range. This is very dif­fer­ent from how stan­dard­ized IQ and achieve­ment tests are con­structed, where scores are cal­cu­lated in rela­tion to age groups that span only sev­eral months.

Third, these find­ings high­light the value of ongo­ing efforts to develop a reli­able objec­tive assess­ment mea­sure for ADHD that is not effected by rel­a­tive age effects. As dis­cussed in a prior issue of Atten­tion Research Update, Quan­ti­ta­tive EEG (qEEG) may be a help­ful tool in this regard. (See Neurofeedback/ Quan­ti­ta­tive EEG for ADHD diag­no­sis)

Finally, the asso­ci­a­tion between rel­a­tive age and risk of diag­no­sis high­lights the impor­tance of sys­tem­at­i­cally reeval­u­at­ing chil­dren each year. As chil­dren develop, the impor­tance of rel­a­tive age on the abil­ity to reg­u­late atten­tion and behav­ior is likely to dimin­ish. For exam­ple, one would expect less dif­fer­ence in the abil­ity to sus­tain atten­tion between younger vs. older 15 year-olds com­pared to younger vs. older 6 year– olds. Thus, if a child was incor­rectly diag­nosed with ADHD because he/she was rel­a­tively young at school entry, and thus less capa­ble than peers of reg­u­lat­ing atten­tion and behav­ior, annual reeval­u­a­tions should iden­tify this as the child moves into later grades.

David Rabiner Attention Research Update– Dr. David Rabiner is a child clin­i­cal psy­chol­o­gist and Direc­tor of Under­grad­u­ate Stud­ies in the Depart­ment of Psy­chol­ogy and Neu­ro­science at Duke Uni­ver­sity.  His research focuses on var­i­ous issues related to ADHD, the impact of atten­tion prob­lems on aca­d­e­mic achieve­ment, and atten­tion train­ing.  He also pub­lishes Atten­tion Research Update, a com­pli­men­tary online newslet­ter that helps par­ents, pro­fes­sion­als, and edu­ca­tors keep up with the lat­est research on ADHD.

Related arti­cles by Dr. Rabiner:

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