"Highly restricted, fixated interests that are abnormal in intensity or focus (e.g., strong attachment to or preoccupation with unusual objects, excessively circumscribed or perseverative interests").The previous version (DSM-IV) also described an:
"Encompassing preoccupation with one or more stereotyped patterns of interest that is abnormal either in intensity or focus."So, there are two aspects of autistic people's interests that are considered unusual: "intensity," the presence of things like intense emotions, drive to engage in the interest, or time spent; and "focus," the absence of interest and attention to other things.
But, are either of these unique enough to autism to be diagnostic? It turns out, children in the general population also have interests that are extremely intense. The jury's still out on focus, as far as I know. My guess is, it won't be unique to autism either. After all, if what you love most is trains and you want to learn and talk about and look at trains all the time, you'll do that whenever you have a choice--and therefore, you'll do other things less.
Researchers on intense interests didn't set out to learn about autistic traits. In fact, they stumbled on these interests while studying other things. Judy DeLoache's team studied how children develop abstract concepts, and Joyce Alexander's team researched children's knowledge about science topics. Alexander's group in particular focused on children with "expertise" in specific topics--children with lots of well-organized knowledge--and noticed that they tended to be extremely interested in their topic of expertise. So, these researchers started examining what these intense interests were like and how common they were.
Judy DeLoache and colleagues  recruited parents of 177 children, ages 11 months to 6 years, who had participated in other studies. They were mostly white and middle class. Parents initially filled out a questionnaire to screen for children who might have intense interests. They were asked only about their child's most intense interest ever. Parents who claimed their child had an intense interest were interviewed by phone. However, a parent's belief that their child had an intense interest was not enough for the researchers. Children's interests were given a score ranging from 1 (moderate interest) to 5 (extremely intense interest) based on duration, the number of different settings where the interest was observed, the number of objects and activities involved in the interest, and the extent to which other people noticed the interest. Only if two raters gave the child a score of 3 or higher were they judged to have an "extremely intense interest."
Joyce Alexander and colleagues [1, 4, 5] did a large longitudinal study, recruiting 215 4 year olds and following up every 2 months for two years. Unfortunately, these children were also mostly white and middle class. Parents were interviewed 13 times about their child's preferred activities during free play, amount of free play time available, preferred TV shows, videos and books, and whether the child had focused interests. Researchers measured the proportion of interviews where children had a focused interest, total number of interest topics over the two years, and the length of the interests.
As part of a neuroimaging study on expertise, Thomas and Karin James  recruited 10 8-12 year olds with intense interests in Pokemon. They used a parent questionnaire based on Alexander's and DeLoache's studies to identify these children. Participants had no record of learning disabilities and had typical scores on the Childhood Autism Screening Test (CAST).
How common are intense interests?
Between 20% and 30% of children seem to have extremely intense interests in one or two topics.
In DeLoache's  sample of 177 children, parents reported that two thirds (116) had an intense interest. Of these, researchers judged 65 to have only moderately strong interests. So 51 children, or 29% of the whole sample, had extremely intense interests.
Joyce Alexander's team reported in 2008  that 21% always had a focused interest during the two years of the study.
These studies suggest that intensity and focus are continuous traits. At one end of the continuum, some children have extremely intense interests focused on one or two topics. At the other end, children never have any focused or strong interest at all. Interestingly, a total lack of focused interests seems rare--only 34% in DeLoache's study , and 10% in Johnson's .
The majority of the population seems to be in the middle, with several different patterns observed:
- Interests are less intense: Similarly focused on 1 or 2 topics, but less intense.
- Interests are less focused: Interests are still highly intense, but are spread across more than 1 or 2 topics.
- Interests are shorter: Interests may still be highly intense but are short--rapidly shifting from one interest to another. For example, while 21% of Alexander's  participants had focused interests, only 7% were interested in only a single topic all 24 months, suggesting many were shifting from one interest to another.
- Combination: Alexander's studies  included some children who had an average of 6 interests lasting up to 10 months, and others with an average of 3 interests lasting up to 22 months. The former group could be considered to have both shorter and less focused interests.
Children's intense interests start early. In DeLoache's study, interests emerged between 3 and 42 months, with the average age being 18 months. Almost 40% of interests were reported to have appeared within the first year, and 90% had developed by age 2. Several parents reported that their children's object of intense interest was among their first words.
Perhaps because these interests appeared so early, almost 80% of the parents had no idea what triggered them; they said these interests had "just always been there."
How long do intense interests last?
Intense interests often last for a long time. DeLoache and colleagues found that these interests lasted from 6 to 36 months, or an average of 22 months. Given that these children were, on average, 35 months old, they had had these interests for most of their lives.
Alexander and colleagues  reported a shorter but still respectable duration--an average of 11 months.
What topics are children intensely interested in?
Topics of interest vary widely,with some being typical for age and gender, and others highly idiosyncratic.
Typical interests included vehicles, balls, books, dolls, and dinosaurs . Interests were often in objects (e.g., tea sets, puzzles, tools) but sometimes in fiction (e.g., the Wizard of Oz) . Interests were often gender-stereotyped , with half the boys interested in vehicles and machines, and almost half the girls interested in things like dress-up, babies, and teasets .
Importantly, unusual interests aren't unique to autism. About 12% of DeLoache's children from the general population also had unusual interests for their age, including :
- The Blue Angels
- Pouring liquids
- Bodies and injuries
- U.S. Presidents
- Brooms, brushes, and mops
- Inventing and building
Who has intense interests?
Children with high IQ. Children with intense interests may have higher IQ than average.
Eight to twelve year olds intensely interested in Pokemon had higher overall, verbal, and nonverbal IQ on the WISC than age peers, as well as higher working memory .
Alexander's team  found that four year olds with intense interests in scientific topics or vehicles had higher receptive vocabulary (on the Peabody Picture Vocabulary Test-III). While vocabulary is not the same thing as verbal intelligence, it is highly correlated in the general population and was used as a proxy for it in this study. Children with intense interests also had higher working memory for shapes (SHAPE SPAN 2).
Another study by the same team  found that children with long-lasting "conceptual" interests in science or vehicles had IQ (Kaufman Brief Intelligence Test) about 1 standard deviation above the normative mean (average standard score=114).
Boys. Most children with intense interests are boys. In DeLoache's study, 75% of the children with intense interests were boys. (By contrast, 43% of the children with moderate interests and 30% of those with no strong interests at all were boys). Researchers also judged boys' intense interests to be more extreme than girls'.
It's not yet clear why there are such strong gender differences. Unfortunately, these researchers cite old work with rather unsophisticated gender concepts by Simon Baron-Cohen as a possible explanation [1, 2, 4]. But better explanations exist.
One clue is that the interest topics themselves are often gender-stereotyped [1, 2, 4]. Even if children are too young to be aware of gender stereotypes, their parents largely control their environment, and may surround their children with gender-stereotyped objects. A boy without access to dressing-up materials is unlikely to develop an intense interest in this activity; a girl without access to Legos would be unlikely to develop an intense interest in them, either.
Another clue is that girls' interests start equally early  and last an equally long time [1, 2]. They simply seem to be focused on different topics or activities--ones that are treated differently by some researchers. Specifically, they were more likely to have interests in creative activities (e.g., drawing), writing, and pretend play, and less likely to have interests in building things or pursuing information about scientific topics and vehicles [1, 4]. To the extent that researchers focus on interests in objects or scientific topics, they may be biased towards finding more boys. So the methods researchers use may also contribute to finding gender differences.
Are intense interests ever a nuisance in the general population?
Parent interviews sometimes hinted that their child's interest could be seen as annoying and was sometimes discouraged .
For example, one little girl's intense interest was in pouring liquids from one container into another. "So constant was this activity that she was banned from visiting the next-door neighbors for repeatedly creating messes by her pouring," DeLoache and colleagues report. Another little boy was fascinated by brooms, mops, rakes, hairbrushes, toothbrushes, and vacuum cleaner attachments. His mother bought him his own and locked the rest in a closet and an out-of-reach cupboard "to keep him from playing with them." One little boy's teacher ended up putting his favorite table with Thomas the Tank Engine trains on it into storage because the boy spent all the time there and stood guard to prevent other children from playing there. Another boy was forbidden to pretend to be a dinosaur at preschool because it was scaring other children.
How does the family influence the development of intense interests?
While parents may not be crucial for generating an interest, their encouragement can help maintain it.
In DeLoache's study, over a third of the intense interests emerged during the first year of life, without any parental encouragement. In fact, some children developed interests their parents found odd, potentially dangerous (e.g., fans), or even disgusting (e.g., injuries and roadkill).
However, parents sometimes give children a gift or engage in an event with them that sparks their interest.  Moreover, the majority of parents do support their children's interests , and this might help them maintain these interests longer. For example, children are more likely to maintain "conceptual" interests (in scientific topics or vehicles) when parents :
- Value consistency and structure in the home.
- Provide lots of opportunities for free play and emphasize educational activities and emphasize communication.
Parents also share their child's interest, or a related one, almost seventy percent of the time . That makes it likely that they and their child discuss the interest and do activities relating to it frequently.
Perceptual nature of early intense interests
Parents of autistic children have noted the perceptual nature of their children's interests, such as the pattern of circles and rotation involved in one child's fascination with clocks, wheels, and planets.
The same may be true in the general population. Children with intense interests in balls initially attended to anything spherical in shape, regardless of size, type of object, or material. Children interested in trains started out fascinated with train tracks, and later with fences and zippers--all sharing a similar pattern of intersecting vertical and horizontal lines .
DeLoache and colleagues seem bemused by why a "meaningless perceptual image" should come to dominate children's lives for such a long time. Children younger than three or four are too young to have the knowledge to determine that these images are meaningless, and to focus on "meaningless" misses the point--this is clearly an aesthetic and emotional experience. The first few chapters of autistic poet Tito Mukhopadyay's book "How can I talk if my lips can't move?" provides a glimpse of how it might feel to be in the grip of an aesthetic experience with everyday objects (in his case, a mirror).
Are intense interests in the general population like special interests in autism?
Young children in the general population have interests that are, statistically speaking, "abnormal in intensity and focus" (given that they are the top 20-30% of the population). These interests are noticeable to others in many contexts, suggesting that they truly dominate children's lives, and may not yet be hidden in the interests of social acceptability. Like autistic interests, these often concern idiosyncratic topics or objects with specific perceptual characteristics. Like autistic interests, intense interests can be viewed as weird or a nuisance to others.
One explanation for these similarities is that at least some of the children with intense interests were on the autism spectrum or at least had strong autistic traits. Most studies of children with intense interests, interestingly, do not report screening for autism spectrum disorder or measuring "autistic traits." One study that did found that 8-12 year olds intensely interested in Pokemon scored normally on the Child Autism Screening Test (CAST), and actually had lower "systematizing" traits than peers on the Systematizing Quotient-Empathizing Quotient test (SQ-EQ).  (Autistic children typically score higher). A single study with 21 participants can't resolve this question. But, if this finding is replicated, it would suggest intense interests aren't solely the product of autism or autistic traits. They really do occur in the non-autistic population, too.
More likely, the predisposition to develop intense interests is a continuous trait. High levels of this trait might be more common in autism, but occur in others, as well1.
Intense interests and special interests look very similar, and they seem to have a positive side and a negative side. The positive side is that they are enjoyable, and seem to motivate people to learn [4, 5]. The negative side is that they can be disruptive , and perhaps consume time and attention that could be spent on other things. Yet, research on intense interests has focused mostly on its positive role in learning, with only one study reporting negative aspects. And while autistic people talk about the joy of special interests and their importance for learning, studies of autistic children prefer to focus on how atypical or disruptive they are [e.g.: 8; 9, 10]--with the exception of this study by Mary-Ann Winter-Messiers, the mother of an autistic son .
So how do we determine whether these are, in fact, the same thing?
First, we must apply the same measures to typically developing and autistic populations. To my knowledge, this has not yet been done yet.2 Furthermore, questionnaires for the general population seem to provide more fine-grained information about the length of children's interests and the way they behave when engaged in their interest.
Second, we would need to start looking at negative aspects of interests in the typical population and positive ones in the autistic population. Parent questionnaires for the general population have generally been neutrally to positively worded, while those for the autistic population have used negative wording and emphasized the weirdness or disruptiveness of the interests. Key questions here: do non-autistic children's interests also tend to get in the way of doing other things? And, can research confirm that autistic children both love and learn a lot from their interests?
One could use an existing general population measure and an existing autistic measure with both groups. Or, better still, one could develop a new set of questions to be used with both groups.
Right now we're assuming that non-autistic interests have more positive consequences (e.g., learning new things) and autistic interests have more negative ones (e.g., socially unacceptable, distracting from schoolwork). This is theoretically possible, but we can't know unless we actually ask both autistic and non-autistic groups the same things.
- Joyce Alexander, Kathy E. Johnson, Mary E. Leibham, & Ken Kelley (2008). The development of conceptual interests in young children. Cognitive Development 23, pp. 324-334. PDF here.
- Judy S. DeLoache, Gabrielle Simcock, & Suzanne Macari (2007). Planes, trains, and automobiles--and tea sets: Extremely intense interests in very young children. Developmental Psychology vol. 43 no. 6, pp. 1579-86. PDF here.
- Thomas W. James and Karin Harman James (2013). Expert individuation of objects increases activation in the fusiform face area of children. Neuroimage vol. 67, pp. 182-192. PDF here.
- Kathy E. Johnson, Joyce M. Alexander, Steven Spencer, Mary E. Leibham, and Carin Neitzel (2004). Factors associated with the early emergence of intense interests within conceptual domains. Cognitive Development vol. 19, pp. 325-343. PDF here.
- Mary E. Leibham, Joyce M. Alexander, Kathy E. Johnson, Carin L. Neitzel, & Fabiola P. Reis-Henrie (2005). Parenting behaviors associated with the maintenance of preschoolers' interests: A prospective longitudinal study. Applied Developmental Psychology vol. 26, pp. 397-414. Abstract here.
- American Psychiatric Association (2000). Diagnostic and Statistical Manual of Mental Disorders (4th ed., text revision). Washington, DC: APA. Autism criteria found here.
- American Psychiatric Association (2013). Diagnostic and statistical manual of mental disorders (5th ed.). Washington, DC: APA. Autism criteria found here.
- Ami Klin, Judith H. Danovich, Amanda B. Merz, & Fred R. Volkmar (2007). Circumscribed interests in higher functioning individuals with autism spectrum disorders: An exploratory study. Research & Practice for Persons with Severe Disabilities vol. 32, iss. 2, pp. 89-100. PDF here.
- Lauren M. Turner-Brown, Kristin S.L. Lam, Tia N. Holtzclaw, Gabriel S. Dicther, & James W. Bodfish (2011). Phenomenology and measurement of circumscribed interests in autism spectrum disorders. Autism vol. 15, iss. 4, pp. 437-456. Full text here.
- Simon Baron-Cohen & Sally Wheelwright (1999). "Obsessions" in children with autism or Asperger syndrome. Content analysis in terms of core domains of cognition. British J. of Psychiatry vol. 175, pp. 484-90. PDF here.
- Mary Ann Winter-Messiers (2007). From tarantulas to toilet brushes: Understanding the special interest areas of children and youth with Asperger syndrome. Remedial & Special Education, vol. 28 no. 3, pp. 140-152. PDF here.
- Michelle Dawson, Laurent Mottron, & Morton Ann Gernsbacher (2008). Learning in Autism. In H.L. Roediger, III. (editor), Cognitive Psychology of Memory. Vol.  of Learning and Memory: A Comprehensive Reference, 4 vols., pp. 749-772. Oxford: Elsevier. PDF here.
- Chloe Jennifer Jordan & Catherine L. Caldwell-Harris (2012). Understanding differences in neurotypical and autism spectrum special interests through internet forums. Intellectual & Developmental Disabilities vol. 50, no. 5, pp. 391-402. PDF here.
1This is purely speculation, but I notice another similarity with autism, as well. Particularly in DeLoache's study, children with intense interests developed their interests independently and often without encouragement from parents. This reminds me of the self-motivated self-teaching that often occurs in autism, which Michelle Dawson, Laurent Mottron and Morton Ann Gernsbacher describe here . In autism, the tendency to teach oneself often comes with difficulty learning from others, which need not be true of non-autistic children who teach themselves. However, I still wonder if there is a relationship between self-motivated intense interests and self-taught learning in general. Is this another relationship common in autism, but existing in the general population?
2 One study does directly compare the "special interests" of neurotypical and autistic adults through internet forums (Wrongplanet for autistic and LiveWire for neurotypical) . However, it analyzed forum content that was already produced, rather than measuring and comparing the intensity of each individual member's interests. It is thus unclear clear whether neurotypicals' interests are intense at all, much less similarly intense and focused to autistic participants'. The researchers were also unable to determine how many of the participants on the "neurotypical" forums were in fact autistic.