6.1.1: Prevalence rates
Sixty-seven percent of the same-sex twin pairs returned the questionnaires (395/589). Of the 194 pairs who did not return the questionnaires, 59 had moved away. This gives a corrected response rate of 75%. The proportion of individual children who scored above the cut-off of 17 on the GDI was 9.2%. The proportion above the cut-off of 37 on the STAIC-Trait was 9.1%. Four percent of the children scored above both of these cut-offs. The correlation between the GDI total score and the STAIG-Trait total score was 0.67. On the GBGL, the proportion of children who scored above the borderline clinical t- score cut-off of 67 was 5.8%, 8.5%, and 2.5% for the anxious/depressed, somatisation and withdrawn syndromes respectively. The proportion of children who scored above the cut-offs on both the STAIG-Trait, and the GDI, and also scored above the borderline cut-off on the anxious/depressed syndrome was 1.1%. The correlations between the anxious/depressed syndrome scores from the GBGL and the GDI and STAIG-Trait scores were .37 and .34 respectively. Agreement between being a case from child report (ie. above the cut-off on either the GDI or the STAIG-Trait) and being a case from parent report (ie. above the borderline clinical cut-off on the GBGL anxious/depressed syndrome was very poor with a kappa value of 0.23. This finding reflects the typical low
agreement between parents and their children in reporting emotional symptoms.
Of the twin pairs in the first stage (N = 529) 22.3% were cases, 66.5% were controls, 4.3% were neither cases nor controls, and 6.8% had missing data. This latter groups largely consisted of families who returned the twin similarity questionnaire when it was sent, but did not return the GDI, STAIC and GBGL when they were subsequently sent out. Sixty-one case pairs were seen in the second stage, which was 51.7% of the identified case pairs. Twenty-nine control pairs were seen which was 8.2% of the available control sample. Thus there were 90 pairs of twins in the second stage of the study.
The Twin Similarity Questionnaire classified 223 pairs as MZ and 172 pairs as DZ. Of these, 192 were male pairs and 203 were female pairs. There were also 104 opposite sex DZ pairs. The age range was 8 to 16 years, with a mean of 11.61, and standard deviation of 2.82.
6.1.2; Creating refined measures of depression and anxiety
As the GDI and STAIG-Trait correlated so highly with one another, it was decided that a factor analysis should be undertaken using the variables from these two scales, with the aim of producing purer anxiety and depression measures. For this analysis the children were treated as individuals rather than as pairs, such that the factors took into account the data from all the children simultaneously. An oblique rotation with a delta of zero was used which allowed the factors to correlate. Factors with an eigenvalue of 1.00 or more were considered. There were 11 that met this criteria. Ten of these had two or more variables loading at a level of at least 0.40 onto them. The factor names and the variable loadings for the variables which loaded at least at the 0.40 level onto them are given in Table 6.1.2a. One exception given in this table is that the variable “worry about schoolwork" which loads 0.39 onto the schoolwork factor is given in the table as this aided interpretation of the factor.
Table 6.1.2a: Factor Analysis of the CD! and STAIC-Trait
Factor Variable Loading
Worry things bother me .49
worry about things happening .42 worry about making mistakes .53
worry too much .54
unimportant thoughts bother me .45
worry about school .49
worry about things that may happen .45
feel troubled .55
worry what others think of me .40
Bad do not do as told .67
get into fights all the time .60
do everything wrong .56
bad all the time .71
bad things are my fault .47
Sleep trouble sleeping .82
hard to fall asleep .82
Decisions cannot make up mind .71
trouble deciding what to do .64
trouble making up mind .79
Lonely don’t want to be with people .62
never have fun at school .59
have no friends .56
nothing is fun .55
Self-esteem look ugly .72
cannot be as good as others .47
do not like myself .51
Schoolwork doing schoolwork is a problem .39
schoolwork is worse than it used to be .41
worry about my parents .49
Sad feel sad .51
feel like crying (GDI) .61
feel like crying (STAIC-Trait) .72
get upset at home .61
Physiological Anxiety heart beats fast .49
hands get sweaty .79
funny feeling in stomach .47
Stable feel alone .44
not sure someone loves me .77
The factor scores were saved for each child and a second-order analysis was conducted. This resulted in two fairly independent factors, one clearly representing depression, the other clearly representing anxiety. The correlation between these two second-order factors was only 0.27. The factor loadings are given in Table 6.1.2b. The loadings of 0.40 or more are in bold type.
Table 6.1.2b: Second Order Factor Analysis of the GDI and STAIC-Trait Factors
First Order Factor Factor Loading on Depression Factor Loading on Anxiety Bad .63 -.06 Self-esteem .59 .07 Lonely .58 .01 Sad .41 .38 Stable .52 -.10 Decisions .29 .43 Physiological Anxiety -.36 .79 Schoolwork -.02 .48 Sleep .18 .45 Worry .33 .47
It should be noted that none of the five factors defining the depression second- order factor loaded onto the anxiety second-order factor at a level of 0.40 or more. The converse was also true. Of the 10 factors from the first stage of the analysis, only three loaded by more than 0.20 onto both factors. These three factors were “sad”, “worry” and “decide”. “Sad” loaded onto anxiety at a level of 0.38 and “worry” and “decide” loaded onto the depression factor at a level of 0.33 and 0.29 respectively, suggesting that these symptoms cannot be entirely separated. However, as noted above, the two second-order factors only correlated with one another at a level of 0.27, and thus represent more refined constructs of depression and anxiety than the total scale scores from the GDI and STAIG-Trait.
It was possible that due to inclusion of data from both members of each twin pair these factors were produced because the variables within them were aetiologically linked to one another. For this reason an identical procedure was carried out using only the first member of each pair, and this analysis produced a very similar final outcome to the original analysis. The correlation between the depression factors from the two analyses was .88, between the two anxiety factors was .83, and between the anxiety and depression factors from this second analysis was .21. This suggests that the second-order factors produced by these two analyses are very similar to one another in their content, and thus it is acceptable to use the factor scores created using the whole sample at once.
These factors represent more pure, refined measures of depression and anxiety, and as such are regarded as the more appropriate constructs for future analyses. The cut-off for cases on each of these factors was chosen to be one standard deviation above the mean. Using this cut-off 14.0% and 16.9% respectively of the children and adolescents were defined as depressed and anxious cases. These rates are somewhat higher than those for the cases defined using the cut-offs chosen for the GDI and STAIC-Trait (9.2% and 9.1% respectively) because the children tended to score lower in the whole sample than in the initial sample from which the CD! and STAIC cut-offs were calculated, so the means for the GDI and STAIG-Trait, and therefore of the depression and anxiety factors were lower in this sample than in the initial sample. Of the 84 children who were cases from the cut-off on the GDI, 94.4% were also cases on the depression factor, and these children accounted for 60.9% of all the cases on the depression factor. Gonsidering the two anxiety measures, 81.1% of the children who were above the STAIG-Trait cut-off were rated as cases on the anxiety factor, but these children were only 43.7% of the whole case group for the anxiety factor. The parent-reported anxious- depressed factor from the GBGL correlated with the depression and anxiety factors by .37 and .24 respectively.
A factor analysis of the symptoms from the GBGL was undertaken to try to retrieve separate depression and anxiety factors from the parent-reported data.
At first only those symptoms included in the internalising syndrome (Achenbach 1991b) were entered into this analysis. As with the child-reported data, an oblique rotation was performed with a delta of zero to allow the factors to correlate. For two reasons this analysis did not produce a useful result. Firstly, there were several symptoms of anxiety and depression which were not part of the internalising syndrome and were therefore not being entered into this analysis (for example some of the fear items, and several of the socialisation items). In addition, three central symptoms did not load at a level of 0.4 or more onto any of the factors produced by this analysis (“unhappy, sad, or depressed", “nervous”, and “anxious”). For this reason a second analysis was undertaken in which all the items which represented symptoms of either anxiety or depression were entered into the analysis. This analysis was once again conducted using an oblique rotation with a delta of zero. The factors were chosen as those with an eigenvalue of more than one. This resulted in an eight factor solution of which six were interpretable (ie. had two or more variables loading on them at a level of 0.4 or more, and which made conceptual sense). These six factors, and the variables that defined them are given in Table 6.1.2c. Two variables are included that load onto the relevant factor by less than 0.4, because these aided the interpretation of that factor.
These factors are not dissimilar to those created in the factor analysis of the CDI and the STAIC-Trait items. However, when these were saved as variables and subjected to a further factor analysis only one second-order factor was produced. When a two factor solution was forced, the grouping of the factors did not make conceptual sense. As can be seen from Table 6.1.2d the factor named nervous loads onto the second-order factor that otherwise represents depression, thus it was not possible to retrieve pure factors of depression and anxiety. This result is in line with the discussion by Achenbach (1991b) who found that parents were unable to distinguish between depression and anxiety in their children.
Table 6.1.2c: Factor Analysis of the Items Representing Depression and Anxiety from the CBCL
Factor Variable Loading
Social Anxiety Anxious .378
Self-conscious .721
Shy .838
Withdrawn Withdrawn .592
Poor peer relationships .755
Likes to be alone .590
Not liked .794
Nervous Bites fingernails .708
Nervous .593
Twitches .668
Sleep Anxiety/Fear Sleep problems .691
Fears - general .446
Nightmares .619
Obsessions .532
Sad Unhappy, sad or depressed .398
Feels unloved .731
Feels persecuted .655
Feels worthless .585
Lonely .579
Cries .415
Schoolwork Fears school .319
Poor schoolwork .853
Poor concentration .777
Table 6.1.2d: Second Order Factor Analysis of the CBCL Factors
First Order Factor Factor Loading on Depression Factor Loading on Anxiety Nervous .460 .124 Sad .632 .110 Schoolwork Problems .795 .223 Withdrawn .586 .063 Sleep Anxiety/Fear .164 .629 Social Anxiety -.071 .847