participants came from the twins early development study ( teds ) , a population - based longitudinal study of all twins born in england and wales from 1994 to 1996 . the present paper is based on data collected from  parents when the twins were approximately 8  ( mean  sd  = 7.89  0.53 ) , 9 ( 9.01  0.29 ) , and 12 ( 11.28  0.70 ) years old , and from teachers when the  twins were approximately 9 ( 9.04  0.29 ) and 12 ( 11.54  0.66 ) years old . zygosity in same - sex twins was established using a parent - rated twin similarity questionnaire ( 60.5% of twins ) and dna genotyping ( 39.5% of twins ) . twin pairs were excluded from the analyses if severe pre- or postnatal complications were reported or if either twin had a severe  medical condition . to retain the full range of variability in autistic traits , children who had suspected or confirmed asd ( n  = 238 ) based on the development and well - being assessment ( dawba ) were included in the analyses when data were available . after exclusions , parent - reported data on both twins were available from 6,280 families at age 8 years , 3,126 families at age 9 years ( the 9-year sample was a subsample of families with twins born between january 1994 and august 1995 ) , and 5,339 families at age 12 years . teacher data were available from 2,663 families at age 9 and 4,405 families at  age 12 . autistic traits were assessed using the childhood autism spectrum test ( cast , formerly the childhood asperger syndrome test ) . the cast was developed as a screening questionnaire for use in non - clinical settings . the published measure consists of 37 items , 31 of which relate to autistic traits ( raters are asked to rate the child on specific behaviors associated with autistic traits ) , the remaining 6 being control questions on general development . at age 12 years , 1 item ( regarding pretend play ) was removed for not being age appropriate . each question was rated  yes  ( 1)/no  ( 0 ) at ages 8 and 12 , and as  not true  ( 0 ) ,  somewhat true  ( 1 ) , or  certainly true  ( 2 ) at age 9 . questionnaires with half or more of the items rated were included in the analyses . only between 0.19% and  2.38% of questionnaires were excluded based on this criterion at any individual age . cast subscale scores were calculated using the method described by  ronald et  al . the cast has good internal consistency : in the present study cronbach 's  ranged from 0.71 to 0.81 , depending on age and rater . it has high sensitivity and  specificity for asd ( both > 95% ) using a cut - point of   15 , as well as high test recently , the cast has been used successfully in epidemiological research to screen for previously undetected cases of asd . at ages 8 and 12 years , the full cast was used . at  age 9 , the 20-item abbreviated version of the cast  was  used . between ages 8 and 9 , the full and abbreviated versions of the cast correlated at 0.63 , which for the same measure would be considered acceptable test  retest reliability across a 1-year interval ( see , e.g. , test  retest reliability of the strengths and difficulties questionnaire ) , supporting the abbreviated version as an equivalent but shorter version of the cast . correlations between teacher and parent ratings were modest at age 9 ( r  = .32 , p  < .001 ) and 12 ( r  = .25 , p  < .001 ) . similar levels of low to moderate agreement between teacher and parent ratings have been reported in  other community - based studies of behavior problems and autistic traits using a variety of established measures ( reviewed by ronald et  al . ) . information on the general principles of the twin design and model fitting can be found in supplement 1 . structural equation modeling ( sem ) analyses of  the  raw twin data were carried out using the software program mx . before modeling , cast scores were first corrected for mean effects of age and sex using regression in spss statistics 19 ( ibm spss , chicago , il ) ,  and then log transformed to remove skew . an unconstrained saturated model accounts for all of the variance and covariance in the observed data without any model assumptions imposed , and will therefore always fit the  data perfectly . however , to meet the theoretical assumptions for modeling the variance components ( additive genetic [ a ] , shared environmental [ c ] , non - additive / dominant genetic [ d ] , and non - shared environmental [ e ] ) and their correlations ( ra , rc , rd , and re ) , the saturated model was constrained so that , when  twin 1 and twin 2 were the same sex , they had the same mean , sd , and correlations . mean , sd , and phenotypic correlations were also constrained to be the same for males and females separately ( e.g. , all male individuals , whether part of an mz , dz same - sex , or dz opposite - sex pair , were assumed to have the same mean , sd , and phenotypic correlation ) . however , males and females were allowed to differ in their mean , sd , and twin correlations . using this constrained saturated model , the phenotypic longitudinal correlations , the twin correlations at each age , and a multivariate quantitative sex - limitation model incorporating equal correlation matrices for males and females was used for both teacher and parent data ( see   scalar sex limitation : correlation approach  in neale et  al . ; note that these investigators use  the  term  scalar sex limitation  to denote a model involving quantitative sex differences ) . using this model , we were able to include data from dz opposite - sex pairs under the constraint that variance component correlations were specified to be the same  for males and females . the inclusion of dz opposite - sex data provided considerable extra power and therefore more accurate parameter estimates . following specification of the parent and teacher models , the causal influence of parameters a , c / d , and e on the longitudinal stability in autistic traits was examined using bivariate estimates ( for details , see supplement 1 ) . participants came from the twins early development study ( teds ) , a population - based longitudinal study of all twins born in england and wales from 1994 to 1996 . the present paper is based on data collected from  parents when the twins were approximately 8  ( mean  sd  = 7.89  0.53 ) , 9 ( 9.01  0.29 ) , and 12 ( 11.28  0.70 ) years old , and from teachers when the  twins were approximately 9 ( 9.04  0.29 ) and 12 ( 11.54  0.66 ) years old . zygosity in same - sex twins was established using a parent - rated twin similarity questionnaire ( 60.5% of twins ) and dna genotyping ( 39.5% of twins ) . twin pairs were excluded from the analyses if severe pre- or postnatal complications were reported or if either twin had a severe  medical condition . to retain the full range of variability in autistic traits , children who had suspected or confirmed asd ( n  = 238 ) based on the development and well - being assessment ( dawba ) were included in the analyses when data were available . after exclusions , parent - reported data on both twins were available from 6,280 families at age 8 years , 3,126 families at age 9 years ( the 9-year sample was a subsample of families with twins born between january 1994 and august 1995 ) , and 5,339 families at age 12 years . teacher data were available from 2,663 families at age 9 and 4,405 families at  age 12 . autistic traits were assessed using the childhood autism spectrum test ( cast , formerly the childhood asperger syndrome test ) . the cast was developed as a screening questionnaire for use in non - clinical settings . the published measure consists of 37 items , 31 of which relate to autistic traits ( raters are asked to rate the child on specific behaviors associated with autistic traits ) , the remaining 6 being control questions on general development . at age 12 years , 1 item ( regarding pretend play ) was removed for not being age appropriate . each question was rated  yes  ( 1)/no  ( 0 ) at ages 8 and 12 , and as  not true  ( 0 ) ,  somewhat true  ( 1 ) , or  certainly true  ( 2 ) at age 9 . questionnaires with half or more of the items rated were included in the analyses . only between 0.19% and  2.38% of questionnaires were excluded based on this criterion at any individual age . cast subscale scores were calculated using the method described by  ronald et  al . the cast has good internal consistency : in the present study cronbach 's  ranged from 0.71 to 0.81 , depending on age and rater . it has high sensitivity and  specificity for asd ( both > 95% ) using a cut - point of   15 , as well as high test recently , the cast has been used successfully in epidemiological research to screen for previously undetected cases of asd . at ages 8 and 12 years , the full cast was used . at  age 9 , the 20-item abbreviated version of the cast  was  used . between ages 8 and 9 , the full and abbreviated versions of the cast correlated at 0.63 , which for the same measure would be considered acceptable test  retest reliability across a 1-year interval ( see , e.g. , test  retest reliability of the strengths and difficulties questionnaire ) , supporting the abbreviated version as an equivalent but shorter version of the cast . correlations between teacher and parent ratings were modest at age 9 ( r  = .32 , p  < .001 ) and 12 ( r  = .25 , p  < .001 ) . similar levels of low to moderate agreement between teacher and parent ratings have been reported in  other community - based studies of behavior problems and autistic traits using a variety of established measures ( reviewed by ronald et  al . ) . information on the general principles of the twin design and model fitting can be found in supplement 1 . structural equation modeling ( sem ) analyses of  the  raw twin data were carried out using the software program mx . before modeling , cast scores were first corrected for mean effects of age and sex using regression in spss statistics 19 ( ibm spss , chicago , il ) ,  and then log transformed to remove skew . an unconstrained saturated model accounts for all of the variance and covariance in the observed data without any model assumptions imposed , and will therefore always fit the  data perfectly . however , to meet the theoretical assumptions for modeling the variance components ( additive genetic [ a ] , shared environmental [ c ] , non - additive / dominant genetic [ d ] , and non - shared environmental [ e ] ) and their correlations ( ra , rc , rd , and re ) , the saturated model was constrained so that , when  twin 1 and twin 2 were the same sex , they had the same mean , sd , and correlations . mean , sd , and phenotypic correlations were also constrained to be the same for males and females separately ( e.g. , all male individuals , whether part of an mz , dz same - sex , or dz opposite - sex pair , were assumed to have the same mean , sd , and phenotypic correlation ) . however , males and females were allowed to differ in their mean , sd , and twin correlations . using this constrained saturated model , the phenotypic longitudinal correlations , the twin correlations at each age , and a multivariate quantitative sex - limitation model incorporating equal correlation matrices for males and females was used for both teacher and parent data ( see   scalar sex limitation : correlation approach  in neale et  al . ; note that these investigators use  the  term  scalar sex limitation  to denote a model involving quantitative sex differences ) . using this model , we were able to include data from dz opposite - sex pairs under the constraint that variance component correlations were specified to be the same  for males and females . the inclusion of dz opposite - sex data provided considerable extra power and therefore more accurate parameter estimates . following specification of the parent and teacher models , the causal influence of parameters a , c / d , and e on the longitudinal stability in autistic traits was examined using bivariate estimates ( for details , see supplement 1 ) . means and sds for each sex   zygosity group can be found in supplement 2 . females scored significantly lower than males on the cast and had significantly smaller sds at  all ages ( p  < .001 ) . also consistent with previous findings , there was a slight but significant ( t4,400  =  6.42 , p  < .001 ) decrease in parent - rated cast scores in males between 8 and 12 years of age ( comparison of mean scores at age 9 with mean scores at other ages was not possible because scores were rated on a 02 scale instead of a 01 scale ) . however , in the present study this  decrease was also seen in females ( t4,952  = 4.43 , p  < .001 ) . parent ratings of autistic traits showed moderately high stability with correlations ranging between 0.59 and 0.67 . teacher ratings showed more modest stability with a correlation of 0.35 for males and 0.26 for females between ages 9 and 12 . the longitudinal correlations were slightly lower in females than in  males ( p  < .01 ) . table  2 contains the within - age and cross - age twin correlations ( for details on the interpretation of twin correlations , see supplement 1 ) . the  within - age ( univariate ) twin correlations can be seen on the diagonal for each sex   zygosity group ( marked in boldface ) . for all age groups , both sexes , and both types of rater , mz correlations were considerably higher than dz correlations , indicating the presence of genetic influences on autistic traits . modest differences between male and female same - sex twin correlations suggested the existence of quantitative sex  differences . dz opposite - sex correlations were generally not substantially lower than dz same - sex correlations , which would have indicated qualitative sex differences . the presence of modest dominant genetic or shared environmental influences was not consistent across ages , sexes and raters , but was indicated in some cases . most noticeable was the suggestion of modest dominant genetic effects for males at ages 8 and 12 years ( but not at age 9 ) in the parent data . in contrast , a small effect of the shared environment was indicated in the teacher data and for females generally . in none of the groups did the mz twin correlation reach 1 , indicating that non - shared environmental influences , including measurement error , played a role . mz cross - twin cross - age correlations were substantially higher than the equivalent dz correlations ( seen in the off - diagonal values in table  2 ) , suggesting that additive genetic influences have an impact on the longitudinal stability of autistic traits . in the parent data , the cross - twin cross - age correlations for mz twins appeared smaller than the within - individual longitudinal ( phenotypic ) correlations , indicating that non - shared environmental factors may play a small role in the stability of autistic traits ; however , in the teacher data , the mz cross - twin cross - age correlation was virtually the same  as the phenotypic correlation . the presence of dominant genetic or shared environmental influences on the longitudinal stability of autistic  traits was not consistent between the different ages . as detailed above , the twin correlations from the teacher scores suggested that an ace model would fit the data best . the parent data were more complex in that the correlations for males at ages 8 and 12 suggested an ade model , whereas the 9-year data for males suggested an ace model . in females , the parent data at all ages indicated that an ace model would fit best . therefore , a hybrid model was implemented to account for the parent data , specifying ade at ages 8 and 12 and ace at age 9 for males , and ace for females at all ages . mz and dz variances  were not significantly different ( p > .05 ) , indicating minimal sibling interaction . the multivariate twin models described in the data analyses section were implemented in mx and are  presented in table  3 . the parameter estimates  for parent - rated data indicated a high contribution of additive genetic factors ( a ) to individual differences in autistic traits at each age for both sexes ( 6876% ) . furthermore , the overlap in these additive genetic influences across ages ( ra )  was substantial : all genetic correlations were >  0.70 . there was a small , but significant , contribution of  shared environmental influences to parent - rated autistic traits at all ages for girls and at age 9 for boys ( 717% ) . in boys , there was a small significant contribution of dominant genetic influences to parent - rated autistic traits at  ages 8 and 12 ; the overlap in these influences  was  very high ( rd  = 1 ) . there was a consistent  contribution of non - shared environmental influences in both sexes at all ages to parent - rated autistic traits ( 1324% ) . the overlap in these influences across age ( re ) was modest ( 0.370.47 ) . the teacher data showed a more modest contribution of additive genetic factors to autistic traits at individual ages ( 3565% ) . the overlap in these influences between 9 and 12 years was considerable ( ra  = 0.58 ) . the effect of shared environmental factors was small and inconsistent , only reaching significance at 9 years in males  and 12 years in females on teacher - rated autistic traits . because the effect of the shared environment was only present at 1 age for each sex , a shared environment correlation could not be estimated . finally , the teacher data suggested a moderate effect of non - shared environmental factors ( 3149% ) at individual ages . however , these factors did not overlap across age ( re close to 0 ) , indicating that different non - shared environmental influences were at play at each age . figure  1 presents , separately for boys and girls , the contribution of each variance component to  the longitudinal stability in autistic traits according to the multivariate twin models . between all ages , and for both raters and sexes , additive genetic factors accounted for most of the  longitudinal stability in autistic traits . in the parent data , there was also a small effect of the non - shared environment on longitudinal stability , as well as a small contribution of dominant genetic effects to the longitudinal stability between 8  and 12 years in boys . the effect of the shared environment on longitudinal stability in autistic traits was non - significant in boys and minimal in girls . in the teacher data , the longitudinal stability in autistic traits the same set of analyses as described for the total cast were carried out on each of the 3 cast subscales ( sis , cis , and rrbis ) . for simplicity , and because the teacher data provided a minimum estimate of longitudinal stability ( see discussion ) , subscale analyses were carried out only on the parent data . the results of these analyses are presented in the supplementary material to  this article : table  s2 presents the longitudinal  phenotypic correlations for each subscale ; tables  s3s5 present the twin correlations , and tables  s6s8 present the modeling results . longitudinal phenotypic correlations were slightly lower for the subscales ( r  = 0.330.62 ) , particularly for sis and rrbis . additive genetic correlations tended to be moderate to  high ( ras >  0.5 ) , the exception being the sis subscale , which demonstrated only modest additive genetic correlations ( ras  = 0.250.31 ) . dominant genetic correlations , where present , were very high ( rds  0.85 ) . non - shared environment correlations were modest for all subscales ( res  = 0.270.39 ) . because of the inconsistent presence of the c component , it was not possible to estimate a shared environment correlation in the subscale models . figures  s1 to s3 present the contribution of each variance component to the longitudinal stability in sis , cis , and rrbis respectively . for all 3 subscales , genetic factors accounted for most of the longitudinal stability , although subscales differed in whether these genetic factors were primarily additive ( cis ) or non - additive ( sis ) . finally , in accordance with findings from  the total scale , non - shared environmental influences played a modest but significant role in longitudinal stability . means and sds for each sex   zygosity group can be found in supplement 2 . females scored significantly lower than males on the cast and had significantly smaller sds at  all ages ( p  < .001 ) . also consistent with previous findings , there was a slight but significant ( t4,400  =  6.42 , p  < .001 ) decrease in parent - rated cast scores in males between 8 and 12 years of age ( comparison of mean scores at age 9 with mean scores at other ages was not possible because scores were rated on a 02 scale instead of a 01 scale ) . however , in the present study this  decrease was also seen in females ( t4,952  = 4.43 , p  < .001 ) . parent ratings of autistic traits showed moderately high stability with correlations ranging between 0.59 and 0.67 . teacher ratings showed more modest stability with a correlation of 0.35 for males and 0.26 for females between ages 9 and 12 . the longitudinal correlations were slightly lower in females than in  males ( p  < .01 ) . table  2 contains the within - age and cross - age twin correlations ( for details on the interpretation of twin correlations , see supplement 1 ) . the  within - age ( univariate ) twin correlations can be seen on the diagonal for each sex   zygosity group ( marked in boldface ) . for all age groups , both sexes , and both types of rater , mz correlations were considerably higher than dz correlations , indicating the presence of genetic influences on autistic traits . modest differences between male and female same - sex twin correlations suggested the existence of quantitative sex  differences . dz opposite - sex correlations were generally not substantially lower than dz same - sex correlations , which would have indicated qualitative sex differences . the presence of modest dominant genetic or shared environmental influences was not consistent across ages , sexes and raters , but was indicated in some cases . most noticeable was the suggestion of modest dominant genetic effects for males at ages 8 and 12 years ( but not at age 9 ) in the parent data . in contrast , a small effect of the shared environment was indicated in the teacher data and for females generally . in none of the groups did the mz twin correlation reach 1 , indicating that non - shared environmental influences , including measurement error , played a role . mz cross - twin cross - age correlations were substantially higher than the equivalent dz correlations ( seen in the off - diagonal values in table  2 ) , suggesting that additive genetic influences have an impact on the longitudinal stability of autistic traits . in the parent data , the cross - twin cross - age correlations for mz twins appeared smaller than the within - individual longitudinal ( phenotypic ) correlations , indicating that non - shared environmental factors may play a small role in the stability of autistic traits ; however , in the teacher data , the mz cross - twin cross - age correlation was virtually the same  as the phenotypic correlation . the presence of dominant genetic or shared environmental influences on the longitudinal stability of autistic  traits was not consistent between the different ages . as detailed above , the twin correlations from the teacher scores suggested that an ace model would fit the data best . the parent data were more complex in that the correlations for males at ages 8 and 12 suggested an ade model , whereas the 9-year data for males suggested an ace model . in females , therefore , a hybrid model was implemented to account for the parent data , specifying ade at ages 8 and 12 and ace at age 9 for males , and ace for females at all ages . mz and dz variances  were not significantly different ( p > .05 ) , indicating minimal sibling interaction . the multivariate twin models described in the data analyses section were implemented in mx and are  presented in table  3 . the parameter estimates  for parent - rated data indicated a high contribution of additive genetic factors ( a ) to individual differences in autistic traits at each age for both sexes ( 6876% ) . furthermore , the overlap in these additive genetic influences across ages ( ra )  was substantial : all genetic correlations were >  0.70 . there was a small , but significant , contribution of  shared environmental influences to parent - rated autistic traits at all ages for girls and at age 9 for boys ( 717% ) . the overlap in these influences across age ( rc ) for girls was minimal . in boys , there was a small significant contribution of dominant genetic influences to parent - rated autistic traits at  ages 8 and 12 ; the overlap in these influences  was  very high ( rd  = 1 ) . there was a consistent  contribution of non - shared environmental influences in both sexes at all ages to parent - rated autistic traits ( 1324% ) . the overlap in these influences across age ( re ) was modest ( 0.370.47 ) . the teacher data showed a more modest contribution of additive genetic factors to autistic traits at individual ages ( 3565% ) . the overlap in these influences between 9 and 12 years was considerable ( ra  = 0.58 ) . the effect of shared environmental factors was small and inconsistent , only reaching significance at 9 years in males  and 12 years in females on teacher - rated autistic traits . because the effect of the shared environment was only present at 1 age for each sex , a shared environment correlation could not be estimated . finally , the teacher data suggested a moderate effect of non - shared environmental factors ( 3149% ) at individual ages . however , these factors did not overlap across age ( re close to 0 ) , indicating that different non - shared environmental influences were at play at each age . figure  1 presents , separately for boys and girls , the contribution of each variance component to  the longitudinal stability in autistic traits according to the multivariate twin models . between all ages , and for both raters and sexes , additive genetic factors accounted for most of the  longitudinal stability in autistic traits . in the parent data , there was also a small effect of the non - shared environment on longitudinal stability , as well as a small contribution of dominant genetic effects to the longitudinal stability between 8  and 12 years in boys . the effect of the shared environment on longitudinal stability in autistic traits was non - significant in boys and minimal in girls . in the teacher data , the longitudinal stability in autistic traits the same set of analyses as described for the total cast were carried out on each of the 3 cast subscales ( sis , cis , and rrbis ) . for simplicity , and because the teacher data provided a minimum estimate of longitudinal stability ( see discussion ) , subscale analyses were carried out only on the parent data . the results of these analyses are presented in the supplementary material to  this article : table  s2 presents the longitudinal  phenotypic correlations for each subscale ; tables  s3s5 present the twin correlations , and tables  s6s8 present the modeling results . longitudinal phenotypic correlations were slightly lower for the subscales ( r  = 0.330.62 ) , particularly for sis and rrbis . additive genetic correlations tended to be moderate to  high ( ras >  0.5 ) , the exception being the sis subscale , which demonstrated only modest additive genetic correlations ( ras  = 0.250.31 ) . dominant genetic correlations , where present , were very high ( rds  0.85 ) . non - shared environment correlations were modest for all subscales ( res  = 0.270.39 ) . because of the inconsistent presence of the c component , it was not possible to estimate a shared environment correlation in the subscale models . figures  s1 to s3 present the contribution of each variance component to the longitudinal stability in sis , cis , and rrbis respectively . for all 3 subscales , genetic factors accounted for most of the longitudinal stability , although subscales differed in whether these genetic factors were primarily additive ( cis ) or non - additive ( sis ) . finally , in accordance with findings from  the total scale , non - shared environmental influences played a modest but significant role in longitudinal stability . the present study is the first large - scale , population - based twin study of genetic and environmental influences on autistic traits longitudinally . consistent with previous evidence , cast scores were significantly and substantially correlated between 8 and 12 years ( table  1 ) . the parent - rated scores showed the highest longitudinal correlations ( r  = 0.590.67 ) . the longitudinal correlations in teacher - rated scores were lower ( r  = 0.260.35 ) . a plausible explanation for the lower stability in teacher ratings is the fact that twins were usually rated by different teachers ( but the same parent ) at each age . nevertheless , it is noteworthy that even with different raters for the same individuals , autistic traits showed a significant degree of stability across age . also replicating previous findings , we found that autistic traits as a whole were moderately to highly heritable at each age . approximately 70% of the variance could be accounted for by additive genetic factors when children were rated by their parents . the equivalent figures for the teacher data were around 55% to 65% , the only exception being the relatively modest contribution of additive genetic effects ( 35% ) to teacher - rated autistic traits in females at age 12 . the smaller contribution of additive genetic influences to variation in teacher - rated autistic traits was matched by somewhat higher contributions of the non - shared environment ( 3149% ) . it is worth noting that measurement error ( included in  the e parameter ) is likely to have been larger in the teacher data because twins were often rated by different teachers , with different levels of knowledge about them . an increase in e puts an upper cap on the a parameter , perhaps explaining some of the discrepancy in heritability estimates between teacher - rated and parent - rated scores . the present study found that the overlap in genetic influences across middle childhood is substantial . in the parent - rated data on total autistic traits , the additive genetic correlation between ages was consistently higher than 0.70 , indicating that most of the additive genetic variance was shared across age . a similar pattern emerged in the teacher data : although it was slightly lower than the parent estimates , the additive genetic correlation of 0.58 between 9 and 12 years suggested considerable overlap in genetic influences across age . furthermore , the bivariate heritability and bivariate environmental estimates revealed that longitudinal stability in autistic traits was largely due to genetic effects ( figure  1 ) . again , longitudinal stability in each trait could be accounted for primarily by genetic effects . they suggest that autistic traits are not just highly stable across middle childhood , but that this stability is caused largely by genetic influences in both boys and girls . this implies that , if these findings generalize to the clinical extreme , stability is a  biological characteristic of autistic symptoms , as opposed to , for example , being due to shared environmental factors . however , the results do not mean that all variance in the stability of autistic traits is caused by genetic influences , nor do they suggest that autistic traits can not be changed . even though the correlation in genetic influences was generally around 0.60 to  0.80 between ages , in no instances did confidence intervals reach 1 , suggesting the presence of age - specific genetic influences as well as genetic influences common across age . notably , the environment shared by the twins , often thought of as the family environment , had virtually no influence on the stability of autistic traits . in contrast , the environment unique to each twin , such as being in different classes at school or receiving differential parental treatment , did seem to play a modest role in the stability of autistic traits . overall , the findings suggest that the development of autistic traits in middle childhood is  characterized mainly by a set of stable genetic  influences , but that , in addition to these influences , new genetic influences as well as modest - to - moderate environmental influences emerge at each age . the specific nature of both these stable and temporary causal influences on autistic traits is  an important topic for future research . the strengths of the present study include its  large sample size and the corroboration of findings across raters . however , the study also has limitations . an abbreviated version of the cast was used at this age , and items were rated on a 02 scale instead of a 01 scale . this might explain why for boys a dominant genetic effect at 8 and 12 years was replaced by a shared environment effect at age 9 . we also can not rule out that this could partly be due to small differences between the full and abbreviated cast . there are also more general limitations of the present study . despite evidence for a continuous distribution of autistic traits in the general population with no clear diagnostic cut - point , the twin design involves important assumptions that need to be met to ensure valid findings , such as the equal environments assumption and the absence of assortative mating ( for discussion , see rijsdijk and sham and plomin et  al . ) . furthermore , gene   environment interaction ( g   e ) can not be estimated in the classical twin design , where any such interaction is included in the a  parameter . finally , the present study relied on parent and teacher ratings . even though it is a strength of the study to have multiple raters , results would benefit from being corroborated using direct observation . future research should endeavor to explore the molecular genetic substrates of the stability in autistic traits , potential g   e interaction , and the causes of stability across the lifespan , which were beyond the scope of this study .