Parental Anxiety, Child Anxiety, and Attention to Threat
Eamon Fulcher
University of The West of England, UK
Andrew Mathews
University of California at Davis, USA
Angie Emler
University College Worcester, UK
Dianne Catherwood
University of Gloucestershire, UK
Marianne Hammerl
University of Regensburg, Germany
Acknowledgements
This research was supported by a Wellcome Trust grant (# 065667) awarded to the first author.
Abstract
Attentional bias for threat may be a precursor to severe anxiety. Since there are moderate concordance rates between children and their parents in anxiety disorders, children with anxious parents may thus show greater attentional vigilance for threat than will children of low anxious parents. Using a modified version of the dot-probe paradigm, maternal self-report anxiety, but not the child's anxiety level, predicted the degree of attentional vigilance for threatening human faces. The result is consistent with the hypothesis that attentional bias contributes to adult anxiety and provides support for a model of adult anxiety which supposes that the key causal agent is the mother's level of anxiety during an individual's childhood.
Parental Anxiety, Child Anxiety, and Attention to Threat
When individuals prone to high levels of anxiety encounter multiple stimuli, which compete for processing resources, they are more likely than others to adopt a threatening interpretation of ambiguous information (e.g., Eysenck, Mogg, May, Richards, & Mathews, 1991) and attend to negatively valenced stimuli (see Mathews & MacLeod, 1994, for a review). This tendency does not seem to depend upon awareness or intent, as it persists even for stimuli that cannot be reported. It may therefore be difficult to control or suppress.
The main method for assessing attention to threat is the dot-probe paradigm, in which attention is measured by the time it takes to respond to a target that appears superimposed on (or replaces) one of two locations where either a threat word or emotionally neutral word has been presented. If individuals are quick to respond to the target, then this may be because their attention was already in that location; and hence, they were reading the word in that location. If a response to the target is slow, then attention is likely to have been captured by the word in the other location. Using this method, attentional bias (preferential attentional capture by stimuli related to threat) has been reported in both clinically anxious populations (MacLeod, Mathews, & Tata, 1986) and in high trait anxious populations (e.g., Broadbent & Broadbent, 1988), the latter effect may be particularly salient when state anxiety is elevated (MacLeod & Mathews, 1988). Low trait anxious individuals, in contrast, typically exhibit no bias and have occasionally shown the reverse effect even when under stress, indicative of avoidance (MacLeod & Mathews, 1988). Research has demonstrated that these results can be reliably extended to more salient and ecologically valid stimuli, such as pictures varying in their threat content (Yiend & Mathews, 2001) and displays of faces varying in their emotional expression (Bradley, Mogg, Falla, & Hamilton, 1998). In addition, attentional bias appears to be disorder-specific (Dalgleish, Moradi, Taghavi, Neshat-Doost, & Yule, 2001).
Research indicates that threat stimuli can be detected pre-attentively (that is, prior to full attention and awareness), and this has been consistently demonstrated in clinically anxious populations using the dot-probe paradigm (Mogg, Bradley, Williams, & Mathews, 1993); and in high trait anxious populations (MacLeod & Rutherford 1992).
It has been suggested that cognitive bias for threat is a contributory cause of clinical anxiety, because it leads to more information about potential dangers being perceived. The underlying mechanism may be a threat evaluation system that serves to drive attention towards threat and towards the negative interpretation of ambiguous information (Mathews & Mackintosh, 1998). This theory has been implemented as a connectionist model (Fulcher, 2001).
The evidence for the causal hypothesis is, however, limited. R esearch has shown that anxiety levels can predict the presence of an attentional bias: it can be present before treatment but disappear after successful treatment (e.g., Mathews, Mogg, Kentish, & Eysenck, 1995) and therapy can have an effect on attentional bias (Lavy, Van Den Hout, & Arntz, 1993); and only a handful of studies have shown that attentional bias can affect anxiety. In these studies, Mathews and Mackintosh (2000) and Mathews & MacLeod (2002) demonstrated that interpretations of emotionally ambiguous words can be biased in either a positive or negative direction by brief training methods. Furthermore, congruent changes in anxiety do occur when the induced bias is subsequently deployed in processing mildly stressful events. Hence emotional processing styles may play a causal role in producing emotional changes. These studies support the notion of a causal relationship between attentional bias and anxiety, but clearly more evidence is needed.
Another way to investigate the causal hypothesis is to study cognitive biases in children. It may be that the existence of a pre-morbid cognitive bias in children predicts later anxiety when the child is exposed to a threatening situation. While a few studies have examined cognitive biases in anxious children, the results have been mixed. Studies that use the dot-probe task tend to find a relationship between anxiety level and attentional bias in children ( Vasey, Daleiden, Williams, & Brown, 1995; Vasey, El-Hag, & Daleiden, 1996). More recently, Hadwin, Donnelly, French, Richards, Watts , and Daley (2003) found that self-report anxiety in children was associated with faster detection of angry schematic faces. These lend support to notion that attentional bias found in anxious adults can be extended to anxiety in children.
Another possibility is that parental anxiety influences the extent of attentional bias in children and that this may pre-date the observable symptoms of anxiety in the child. While there is evidence for a genetic component to anxiety disorders (Jang, Stein, Taylor, & Livesley, 1999; Stein, Jang, & Livesley, 1999), there is no evidence that attentional bias develops experientially though parent-child interactions. According to Kindt et al. (2003), attentional bias may develop early in most children, who then tend to unlearn this bias with age. Those who fail to unlearn attentional bias may go on to develop an anxiety disorder. This view is not inconsistent with the notion that failure to unlearn an attentional bias for threat is due to parental influences, such as through vicarious learning. If this is the case, then an important predictor of attentional bias in young children would be the degree of parental anxiety. Indeed, parental reaction to a traumatic event, for example, can influence the development of post traumatic stress disorder (PTSD) in children (Foa, 2000). Furthermore, Muris, Steerneman, Merckelbach, and Meesters (1996) found that children of mothers who tend to verbalise their fears regularly had an elevated number of fears of specific situations and stimuli, and that these fears were related to the mother's fears. Interestingly, these fears in the child were unrelated to those of the father's. Other research suggests that the responses of a caregiver can influence a child's fear reaction (Klinnert, Campos, Sorce, Emde, & Sredja, 1983).
If parental anxiety does influence child anxiety then there should be a strong concordance between child and parent anxiety disorders. Last, Phillip, and Statfeld (1987) studied the lifetime psychiatric history of mothers of 58 children with separation anxiety disorder, over-anxious disorder, or both, and compared them with a control group. Mothers of children with anxiety disorder had an 83% lifetime rate of anxiety disorders, compared with a rate of 40% for the controls. In addition, 57% of the mothers of anxious children were suffering from an anxiety disorder at the time of the study, compared with 20% of the control mothers. The results of this study would seem to suggest strong familial influences in the development of anxiety disorder, but the underlying mechanisms are unclear. Some possibilities include an overprotective parenting style (Rapee, 1997), shared life experiences (Craske, 1999), or as hypothesised here, acquired attentional bias through a form of vicarious learning from one's parents or carers. In a study by Field, Argyris, and Knowles (2001) it was found that vicarious learning in children, in the form of negative information given by adults about a fictitious character, can lead to increased fear beliefs. Other studies have shown that vicarious fear conditioning (by observing others experiencing a mild shock or by being informed about shock contingencies) can be as effective as Pavlovian conditioning in which shocks are actually delivered (Olsson & Phelps, 2004) and that a mother's fearful facial expression can act as strong unconditioned stimuli for their young children (Gerull & Rapee, 2002). Taken together, the studies reported here imply that anxiety can be transmitted from parent to child through verbalised negative information.
A finding that is consistent with the causal hypothesis was obtained by Moradi, Neshat-Doost, Teghavi, Yule, and Dalgleish (1999) who examined attentional bias in children of mothers with PTSD. They found that in comparison to a control group, children of mothers with PTSD showed greater attentional interference from threat-related words than from emotionally neutral words. Relating this finding to the causal hypothesis, the study is consistent with the view that attentional bias is one consequence of the emotional background in the family and that the bias may predate manifest anxiety. However, since anxiety levels were not measured in the children, the study does not rule out the possibility that high levels of anxiety were already present in the children of mothers with PTSD. To test the causal hypothesis further, it would need to be shown that attentional bias is present before the observable symptoms of anxiety. Therefore, the aim of the present study is to assess whether maternal anxiety is associated with attentional vigilance for threatening information in children with few symptoms of anxiety. However, if parental influences in anxiety are as considerable as previous research suggests then we might expect a hgh correlation between the mother and child's anxiety levels. If so, then attentional bias may be better predicted by the anxiety level of the child.
Method
Design
In this study, a ttentional preference was assessed by presenting threatening and non-threatening adult human faces in a modified version of the dot-probe paradigm. To ensure visual engagement with the material, the task was to search for a target (picture of a ladybird) that appeared 1s later in a semi-random location on the screen. The direction of attention was assessed by analysing reaction times to the appearance of the target.
Participants
Participants were 97 children in Year 3 Primary of ages 7-8 years. These were recruited from 10 local schools, including one independent school (urban), four state schools (urban), and five state schools (rural). There were 40 boys and 57 girls. Recruitment from each school was via a random sampling procedure, where each child had an equal chance of being selected. Information sheets that conveyed an outline of the task and two consent forms (one for the main caregiver and one for the child) were sent out to all parents of the class. Participating children were then selected from the sample of those returning fully completed consent forms before a fixed deadline. The experimental procedure was approved by the Ethics Committee at University College Worcester and The Worcestershire NHS Ethics Committee. Donations were made to each participating school, and the amount was based on the number of children tested. Each child chose a small toy as a reward for their participation.
Materials
Beck Anxiety Inventory (BAI). The Beck Anxiety Inventory (BAI; Beck, Epstein, Brown, & Steer, 1988) was distributed to all parents of selected children at participating schools. Parents or guardians (both male and female carers where possible) responded to 21 items. Each item is descriptive of subjective, somatic or pain related symptoms of anxiety. Each question asks how much each of the symptoms had bothered them in the past week.
Multidimensional Anxiety Scale for Children (MASC). This scale has been developed for and tested on children between the ages 8 to 16 (March, Parker, Sullivan, & Stallings, 1997). It is a 39-item questionnaire and factor analysis reveals that it has four subscales that measure physical symptoms ( tension, restlessness, and somatic/autonomic symptoms; PS) , harm avoidance ( perfectionism and anxious coping; HA) , social anxiety ( humiliation/rejection and public performance fears; SA) , and panic/separation anxiety (PSA), and also provides a total anxiety score (sum of the subscales). It has been shown that girls score significantly higher on all factors than boys; and that parent-child agreement is moderate, but highest for easily observable symptoms (March et al., 1997). This questionnaire was completed by parents (in all but one case by the mother) of the children selected to participate.
MASC-10. This is a shortened version of the 39-item MASC, and it was completed by teachers on a sample of the children that took part in the study.
Stimuli. Eight grey-scale photographs of adult faces were used as stimuli (each of four models was photographed twice: once with an angry expression and once with a smile). These photographs were used in a previous unpublished study in which they were selected from a large sample on the basis of the threat rating of the angry expression and the pleasantness rating of the smiling expression by a panel of raters. Those pairs that provided the highest ratings for both expressions were selected. The pictures measured 12 cm wide by 9 cm long and were presented 14 cm apart.
Procedure
The participant was seated centrally in front of the screen, approximately 60 cm from the TFT flat-screen. Instructions were presented on-screen and these were also read aloud by the experimenter. Following the instructions, the attentional trials began. A yellow cross appeared in the centre of the screen for 2 seconds, at which participants were instructed to fixate. The cross then disappeared, to be replaced by a pair of pictures, one on the left and one on the right of the screen. Superimposed on one of these pictures after a short ISI of 500 ms was a small image of a Ladybird (Ladybug). The participant's task was to indicate on which of the pictures the target image had appeared (by pressing A on the keyboard for the left picture and 5 on the numeric keypad of the keyboard for the right picture). Participants were presented with 4 practice trials initially (using two smiling adult faces). Participants were presented with four smiling/frowning face pairs (that is the same model photographed with an angry expression on one side of the monitor and with a happy expression on the other side of the monitor), twice each (controlling for left and right positions of the expressions) and over 4 blocks of trials. The order of the presentation of the pairs within a block was randomized. Thus, there were 32 attentional trials in all (4 models x 2 within-trial poresentations x 4 blocks of trials).
Results
Beck Anxiety Inventory
Ninety-five out of 97 mothers (or female carers) completed the BAI and their scores range from 0 to 26, with a mean of 6.65 ( SD = 5.2). Seventy-three fathers (or male carers) completed the BAI, and their scores range from 0 to 17, with a mean of 4.52 ( SD = 4.62). An independent t -test revealed that mothers had significantly higher reported BAI scores than fathers, t (71) = 2.636, p < 0.01.
Multidimensional Anxiety Scale for Children
Ninety-five MASC questionnaires were completed on behalf the children. The total MASC scores range from 36 to 75, with a mean of 50.78 ( SD = 7.99).
Validity of MASC for 7-year Olds
The self-report MASC was developed and has been validated for 8- to 16-year olds. Since not all Ps had reached age 8 and since mothers completed the form on behalf of their children, we attempted to assess the reliability of the mother's appraisal by asking form teachers to complete, for a random sample of 14 children, the MASC- 10, a short form of the full MASC. The MASC-10 as completed by the form teacher had a statistically significant positive correlation with the overall MASC score, r (13) = 0.613, p < 0.05, lending support to the reliability of the MASC for this sample. However, since March et al. (1997) obtained only moderate agreement between the mother and child on this measure, we should be cautious about the validity of the MASC as representing an accurate record of the child's anxiety.
Attentional Measures
In preparing the analysis we considered treating the independent variables dichotomously through the method of the median split or by consulting the norms for each measure. However, we chose not to do this because of the statistically dubious practice of converting a continuous variable into a dichotomous variable when there is no theoretical reason to do so, and also because the range of scores in any group defined by a median split could readily be different for different samples. For these reasons, most of the subsequent analyses use multiple regression with the subject variables entered as potential predictor variables of the dependent measures.
Reaction time. We computed a reaction time index (RT Index) based on the mean reaction times to targets in the location of angry or smiling faces (after excluding 4.5% of reaction times that were response errors). We computed an index as the extent to which visual engagement was preferred for negative versus positive stimuli. The RT Index is the mean reaction time when the target is superimposed on negative faces subtracted from the mean reaction time when the target is superimposed on positive faces. An RT Index greater than zero implies more attention to negative faces and an RT Index less than zero implies attention to positive faces.
We entered the RT Index as the dependent variable into a stepwise multiple-regression analysis, with mother's BAI, father's BAI, and each of the four MASC subscales as predictor variables.
There was a highly statistically significant association between several independent variables and the RT Index [Multiple R = 3.92; F (3, 104) = 6.295, p < 0.001]. Together, harm avoidance (HA), father's BAI, and mother's BAI accounted for 12.9% of the variance. HA and father's BAI were negatively associated with the RT Index, while mother's BAI was positively associated with the RT Index. The regression coefficient ß for harm avoidance was -5.709 ( t = -2.526, p < 0.013); for father's BAI ß was -12.61 ( t = -3.067, p = 0.003); and for mother's BAI ß was 8.19 ( t = 2.34, p = 0.021). In sum, HA and father's BAI predicted attentional avoidance of photographs of angry faces, whereas mother's BAI predicted attentional vigilance.
Discussion
The most important finding is that maternal anxiety level is associated with attentional vigilance for threatening faces. None of the MASC subscales were associated with attentional vigilance. This result is consistent with the main hypothesis that maternal anxiety would predict attentional vigilance of threatening information better than the child's level of anxiety. The obvious implication is that attentional bias develops or is maintained through maternal influences. Another implication is that attentional bias may predate the symptoms of anxiety, and this is consistent with the view that cognitive processes in dealing with threat are acquired before the physical symptoms of anxiety appear. This study therefore extends the findings of previous research, such as the study of Moradi et al. (1999) who also found attentional bias to be predicted by a measure of maternal anxiety. However, the MASC measures were taken from the mother's observations and not from those of the child and although there was strong mother and teacher agreement on the MASC, the data in this study may represent the expectations of parents and teachers rather than actual anxiety levels of the children.
A second finding was that the fathers' anxiety, together with the harm avoidance measure on the MASC, predicted attentional avoidance. This is not inconsistent with the previous result if one assumes that children scoring high on the harm avoidance measure use attentional avoidance of threatening material as a coping strategy. We could further speculate that this measure is related to high paternal anxiety, which may be perceived in threatening ways. The evidence reviewed earlier suggests that the mother's anxiety is more closely related to the child's than is the father's; especially in terms of its severity and in the specific kinds of fears they share. The association between paternal anxiety and attentional avoidance does not therefore threaten the validity of the observed relationship between maternal anxiety and attentional vigilance.
One limitation of the study is that we did not consider any history of clinical anxiety and whether any child or parent had undergone any form of treatment for anxiety. Furthermore, most scores on the measures are skewed towards the lower end of the scale. These aspects of the study may make it difficult to generalise the findings to a clinical population. Equally, it may be that the effect observed here would be more pronounced with clinically anxious mothers. If so, then the results of the present study are quite remarkable given the low levels of self-reported anxiety.
Another limitation is that in the absence of a longitudinal data, it is difficult to draw firm conclusions about the extent to which this study is a test of the causal hypothesis. In a future study, it may be fruitful to measure attentional bias in both the parent and the child in order to provide more direct evidence that attentional bias is acquired from the parents. In addition, a longitudinal study could assess both attentional and anxiety measures to examine their development over time and thereby to identify more precisely how they might be related.
In future work in this area, our aim is to examine the acquisition of attentional bias in children within the laboratory setting. In a previous study, we used evaluative conditioning to assign valence to previously novel stimuli (Fulcher, Mathews, Mackintosh, & Law, 2001). The stimuli with newly acquired valence were then presented in a version of the dot-probe task. Stimuli that had been paired with threatening information captured more attention in participants scoring high on a measure of anxiety than stimuli that had been paired with pleasant stimuli. The study demonstrates how the range of stimuli capable of attracting attention in anxiety-prone individuals can be extended through experience. It would be interesting to examine whether this effect is present in children with anxious mothers. It would also be interesting to discover if a bias in evaluative conditioning exists in anxiety, such as a relationship between anxiety level and the magnitude of the learning effect with negative, but not positive, associations. Given the recent research findings of the power of observational and instructional fear conditioning, it may also be fruitful to examine a vicarious form of evaluative learning and its potential relationship with anxiety.
In conclusion, we have found evidence of an attentional bias for threat in children with anxious mothers and hence evidence consistent with the hypothesis that attentional bias contributes to adult anxiety. This also provides support for a model of adult anxiety which supposes that the key causal agent is the mother's level of anxiety during an individual's childhood.
References
Beck, A. T., Epstein, N., Brown, G., & Steer, R. A. (1988). An inventory for measuring clinical anxiety: Psychometric properties. Journal of Consulting and Clinical Psychology, 56 , 893-897.
Bradley , B. P., Mogg , K., Falla , S. J., & Hamilton , L. R. ( 1998 ). Attentional bias for threatening facial expressions in anxiety: Manipulation of stimulus duration. Cognition and Emotion, 12, 737-753.
Broadbent, D., & Broadbent, M. (1988). Anxiety and attentional bias: State and
trait. Cognition and Emotion, 2, 165-183.
Craske, M. G. (1999). Anxiety disorders: Psychological approaches to theory and treatment. Boulder , CO : Westview.
Dalgleish, T., Moradi, A.R., Taghavi, M.R., Neshat-Doost, H.T., & Yule, W. (2001). An experimental investigation of hypervigilance for threat in children and adolescents with post-traumatic stress disorder. Psychological Medicine, 31 , 541–547.
Eysenck , M. W., Mogg , K., May , J., Richards , A., & Mathews , A. (1991). Bias in interpretation of ambiguous sentences related to threat in anxiety. Journal of Abnormal Psychology, 100, 144-150.
Field, A. P., Argyris, N. G., and Knowles, K. A. (2001). Who's afraid of the big bad wolf: a prospective paradigm to test Rachman's indirect pathways in children. Behaviour Research and Therapy, 39 , 1259–1276.
Fulcher, E. P. (2001). Neurons with attitude: A connectionist account of human evaluative learning. In S. Moore & M. Oaksford (Eds.), Emotional cognition: From brain to behaviour (pp. 75-109) . Amsterdam : John Benjamins.
Fulcher, E. P., Mathews, A., Mackintosh, B., & Law, S. (2001). Evaluative learning and the allocation of attention in anxiety. Cognitive Therapy and Research, 25, 261-280.
Foa, E. (2000). Psychological treatment of post-traumatic stress disorder. Journal of Clinical Psychiatry, 61 , 43-51.
Gerull, F.C., & Rapee, R.M. (2002). Mother knows best: Effects of maternal modeling on the acquisition of fear and avoidance behaviour in toddlers. Behaviour Research and Therapy, 40, 279–287.
Hadwin, J. A., Donnelly, N., French, C. C., Richards, A., Watts , A., and Daley, D. (2003). The influence of children's self-report trait anxiety and depression on visual search for emotional faces. Journal of Child Psychology and Psychiatry, 44 , 432.
Jang, K. L., Stein, M. B., Taylor, S., & Livesley, W. J. (1999). Gender differences in the etiology of anxiety sensitivity: A twin study. Journal of Gender Specific Medicine, 2, 39-44.
Kindt, M., Bögels, S., & Morren, M. (2003). Processing bias in children with separation anxiety disorder, social phobia, and generalised anxiety disorder. Behaviour Change, 20, 143-150.
Kindt, M., & Van den Hout, M. (2001). Selective attention and anxiety: A perspective on developmental issues and the causal status. Journal of Psychopathology and Behavioral Assessment, 23 , 193-202.
Klinnert, R. D., Campos, J. J., Sorce, J. F., Emde, R. N., & Sredja, R. (1983). Emotions as behaviour regulators. In R. Phitchik, & H. Kellerman, The emotions , vol. 2. New York : Academic Press.
Last, C. G., Phillips, J. E., & Statfeld, A. (1987). Childhood anxiety disorders in mothers and their children. Child Psychiatry and Human Development, 18 , 103-112.
Lavy, E., Van den Hout, M., & Arntz, A. (1993). Attentional bias and spider phobia: Conceptuel and clinical issues. Behaviour Research and Therapy, 31, 17-24.
Mackintosh, B. & Mathews, A. (2003). Don't look now: Attentional avoidance of emotionally-valenced cues. Cognition & Emotion, 17 , 623-646.
MacLeod, C., & Mathews, A. (1988). Anxiety and the allocation of attention to threat. Quarterly Journal of Experimental Psychology: Human Experimental Psychology, 40A, 653-670.
MacLeod, C., Mathews, A., & Tata, P. (1986). Attentional bias in emotional disorders. Journal of Abnormal Psychology, 95, 15-20.
MacLeod, C., & Rutherford, E. (1992). Anxiety and the selective processing of emotional information: Mediating roles of awareness, trait and state variables, and personal relevance of stimulus materials. Behaviour Research and Therapy, 30, 479-491.
March, J. S., Parker, J. D. A., Sullivan, K., Stallings, P. K., & Conners, C. K. (1997). The multidimensional anxiety scale for children (MASC): Factor structure, reliability, and validity. Journal of the American Academy of Child and Adolescent Psychiatry, 63 , 554-565.
Mathews, A., & Mackintosh, B. (1998). A cognitive model of selective processing in anxiety. Cognitive Therapy and Research, 22, 539-560.
Mathews, A., & MacLeod, C. (1985). Selective processing of threat cues in anxiety states. Behaviour Research and Therapy, 23 , 563-569.
Mathews, A., & MacLeod, C. (1994). Cognitive approaches to emotion and emotional disorders. Annual Review of Psychology, 45 , 25-50.
Mathews, A. & MacLeod, C. (2002). Induced processing biases have causal effects on anxiety. Cognition & Emotion, 16 , 310-315.
Mathews, A., Mogg, K., Kentish, J., & Eysenck, M. (1995). Effect of psychological treatment on cognitive bias in generalized anxiety disorder. Behaviour Research and Therapy, 33 , 293-303.
Mogg, K., Bradley, B., Williams, R., & Mathews, A. (1993). Subliminal processing of emotional information in anxiety and depression . Journal of Abnormal Psychology, 102 , 304-311.
Muris, P., Steerneman, P., Merckelbach, H., & Meesters, C. (1996). Parental modelling and fearfulness in middle childhood. Behaviour Research and Therapy, 34 , 265–268.
Olsson, A. & Phelps, E. A. (2004). Learned fear of ‘‘unseen'' faces after Pavlovian, observational, and instructed fear. Psychological Science, 15 , 822-828.
Rapee, R. M. (1997). Potential role of childrearing practices in the development of anxiety and depression. Clinical Psychology Review, 17 , 47-67.
Stein, M. B., Jang, K. L., & Livesley, W. J. (1999). Heritability of anxiety sensitivity: A twin study. American Journal of Psychiatry, 156 , 246-251.
Vasey, M. W., Daleiden, E. L., Williams, L. L., & Brown, L. M. (1995). Biased attention in childhood anxiety disroders: A preliminary study . Journal of Abnormal Child Psychology, 23 , 267-279.
Vasey, M. W., El-Hag, N., & Daleiden, E. L. (1996). Anxiety and the processing of emotionally threatening stimuli: Distinctive patterns of selective attention among high- and low-test-anxious children . Child Development, 67 , 1173-1185.
W atts , F. N., McKenna, F. P., Sharrock, R., & Trezise, L. (1986). Colour naming of phobia-related words. British Journal of Psychology, 77 , 97-108.
Yiend, J., & Mathews, A. (2001). Anxiety and attention to threatening pictures. Quarterly Journal of Experimental Psychology: Human Experimental Psychology, 54A, 665-681.