Few people in modern societies are untouched by internalising problems, a broad term that refers to emotional distress and encompasses the spectrum of emotional symptoms of anxiety and depression. Although in clinical practice anxiety and depression disorders are seen as multiple, distinct diagnoses, empirical evidence shows high overlap between them and supports use of the broad term internalising problems [1, 2]. The World Health Organization (WHO) predicts that, by 2030, internalising problems will be second only to HIV/AIDS in burden of disease .
Mental health problems affect 1 in 7 school aged children , although they can occur in children of all ages. Internalising (emotional) and externalising (behavioural) problems are among the most common difficulties of early childhood, affecting approximately 15% of those aged 18 months to 5 years [5–8]. Australian community studies have recently confirmed this high prevalence and stability of internalising symptoms across early to mid childhood (e.g. Pearson r's =.53 to.63) [9, 10]. By the time internalising disorders are detected problems can be severe and treatment effectiveness can be limited [11, 12].
Early internalising problems often have longer-term consequences, with many adult problems having early roots in childhood [4, 13, 14]. Convergent evidence from prospective and retrospective studies confirms that internalising problems often persist into adolescence and then into adulthood [4, 15–24]. Their impacts extend beyond mental health to adult relationships, employment opportunities, and even early mortality. For example, in the British National Child Development Study (N = 11,142), internalising problems at ages 7-11 years were predictive of higher mortality by age 45 (OR 1.20, 95% CI 1.06-1.35) .
Evidence suggests that early intervention is key to producing a positive impact because it may be more difficult to influence developmental outcomes later in childhood [25, 26]. Though limited, the evidence also supports the cost-effectiveness of intervening early in development [25, 27, 28]. While emotional functioning continues to develop into adulthood, the early years constitute a window of opportunity for effective mental health promotion in at-risk children. The application of prevention to internalising problems in early childhood is still in its infancy . In 2009, Bayer and colleagues conducted a systematic review of early interventions (age 0-8 years) to improve child mental health. This review found a paucity of randomised controlled trials aiming to reduce internalising problems in community settings .
Rationale for the proposed Cool Little Kids population-level study
The strongest precursor of internalising problems in young children is temperamental inhibition, manifested as fearfulness and a tendency to withdraw from new situations [29, 31–34]. Additional known risks are harsh and overprotective parenting, and parent internalising problems [9, 10, 34–39]. Together, these account for up to 45% of the variance in early childhood internalising symptoms [9, 10, 35].
The only randomised trials testing a parenting prevention model in inhibited preschool children were conducted by Rapee. Rapee's Cool Little Kids program is the first (and, thus far, only) effective early childhood prevention program for internalising disorders [30, 40–42]. Targeting child inhibition and overprotective parenting, this parenting program aims to help preschool children become resilient to situational fears and abstract distressing worries. It teaches parents strategies to modify their preschool child's fear and distress, as well as their own (if relevant), based on standard principles for treating internalising disorders in children and adults [11, 42].
Two successful efficacy trials of the Cool Little Kids program have been reported. Rapee's first trial  recruited 146 children aged 4 years with temperamental inhibition, measured by parent-report questionnaire (>85th percentile) and intensive laboratory observation. Intervention parents received a university-based prevention program from teams of two clinical psychologists offering six group sessions designed to reduce overprotective parenting in response to early fearful behaviour. By age 5 years, the intervention children had developed significantly fewer anxiety disorders than controls (50% vs. 64%). These effects were even larger by age 7 (40% vs. 69%) . Rapee's second study  recruited 71 inhibited preschoolers whose parents themselves had internalising disorders. The intervention group received an eight-session version of the program which extended to focus on parent anxiety as well as overprotective parenting. Six months later, the intervention children had substantially fewer internalising disorders, diagnosed in only 53% of the intervention group compared to 93% of controls.
Rapee's two efficacy trials are at the cutting edge of prevention research and have major potential public health implications. Population conclusions, however, are precluded by their sample bias (university location and self-selection by advertisement) and the labour-intensive laboratory observation methods used for selection. The unaddressed challenge is to determine 'real world' effectiveness across an entire population. We report the protocol for the next step - to conduct a population-level translational randomised trial.
Aims and hypotheses
The aims of this trial are to (a) determine the balance of benefits and harms of systematically screening preschoolers for temperamental inhibition and of a parenting intervention program offered to those at risk, (b) examine the impacts on child internalising problems at school entry, and (c) evaluate cost-effectiveness.
We hypothesise that children whose parents enter the program will do better one and two years after baseline (the first two years of school for most) than 'usual care' control children on the outcomes: i) fewer children with internalising disorders, ii) lower mean scores on early child internalising symptoms, iii) lower mean scores on harsh and overprotective parenting, iv) lower mean scores on parent internalising problems. We anticipate that the prevention program will be acceptable and cost-effective.