The significance of the first years of living in determining mental health later in life has been acknowledged since the early psychoanalytic approaches to the understanding of mental illness delivered by Sigmund and Anna Freud, Rene Spitz and John Bowlby [1,2]. Empirical research since then has documented the early childhood origin of most mental disorders seen in older children and adolescents [3,4]. Birth cohort studies have provided evidence on the interplay of genes and environment from early stages of prenatal life [5-7] and the longterm influences of pre-and perinatal risk factors [3,8,9]. Also, studies of general population birth cohorts have identified infancy precursors of mental disorders diagnosed in preschool and school age [10-13].

Epidemiological research from the last decades has documented the public health burden of mental health problems and disorders in older children, adolescents and adults [2,14-17], and highlighted the needs of preventive intervention in early childhood [18]. However, the knowledge on the developmental epidemiology of mental disorders in early ages is still limited, and the scientific foundation of preventive intervention correspondingly scanty [1,19,20].

Until recently, epidemiological research in mental disorders in younger children has been hampered by limitations in measures of diagnostic assessment. Over the last decades, however, validated measures have increasingly been used, both in studies of clinically referred childen (e.g. Lavigne et al. [21]; Frankel et al. [22]) and in community studies (e.g. Skovgaard et al. [23], Skovgaaard et al. [24], Wichstrøm et al. [25]), and the existence of impairing clinical syndromes in children down to the ages of 0-3 years has been established [26], as reflected in the international classification schemes: The ICD-10 Classification of Mental and Behavioral Disorders [27]; the Diagnostic Statistical Manual of Mental Disorders, DSM-5 [28] and the Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood, DC: 0-3 [29] and DC:0-5 [30].

The first general population study that includes the range of mental disorders in children below the age of two years was based on The Copenhagen Child Cohort CCC2000 [9,24]. In a random sample (N=211) nested in the cohort (N=6090) of 1½ year old children from the general population in Copenhagen County (the suburbs of Copenhagen), the prevalence of any ICD-10 mental disorder was found to be 16.0%, 95% CI (11.9, 22.1). Disorders within the area of emotions, behaviour, eating and sleep were the most common among these children, whereas developmental disorders and disorders of hyperactivity and inattention were found in 2.8 % CI (1.1, 6.1) and 2.4 % CI (0.8, 5.4), respectively [24]. The diagnostic distribution and patterns of risks found in children aged 1½ years were found similar to results from community studies of older children [3]. Also, neuro-developmental disorders were found associated to pre-and perinatal biological risk factors, whereas psycho-social risks in the family were highly asociated with emotional, behavioural, eating and sleep disorders [24]. Within the first years of living, the CCC2000 study identified specific infancy risk trajectories of neuro-developmental problems and problems of parent-child relations andemotional and behavioural disorders [31,32], findings in line with results from other recent birth cohorts [11-13].

The results from the CCC2000 were included in a recent research review with a meta-analysis of the international prevalence and comorbidity of mental disorders in children between 1 and 7 years [20]. The review covered the period 2006-2020 and included ten studies from eight countries (USA, Iraq, Brazil, Romania, Spain, the Netherlands, Norway, Denmark) with data on a total of 18,282 children. The overall pooled prevalence of mental disorders was 20.1%, 95% CI (15.7, 25.4). The review included only diagnoses that were reported in at least four studies, which left pervasive developmental disorders/autism-spectrum disorders, specific developmental disorders (e.g. disorders of language development) and general developmental disorders (e.g. intellectual disability) out of the analyses. Among the included disorders, oppositional defiant disorder had a pooled prevalence of 4.9%, 95% CI (2.5, 9.5); attention-deficit hyperactivity disorder 4.3%, 95% CI (2.5, 7.2); anxiety disorders 8.5%, 95% CI (5.2, 13.5), and depressive disorders 1.1%, 95% CI (0.8, 1.6). Comorbidity, e.g co-ocurrence of two or more of any of the included mental disorders, was estimated at 6.4%, 95% CI (1.3, 54.0). Based on their review, the authors concluded that the epidemiology of mental disorders in children younger than 7 years is still under-researched [20].

Denmark is a country of unique possibilities regarding epidemiological research, mainly due to the potentials of population registries that cover all citizens across life and includes all patient contacts to public hospitals with systematic recordings of diagnoses [33]. Contact to public hospitals is free of charge for the patient, which reduces social selection of referrals. Several ICD-10 diagnoses of mental disorders used to children and adolescents have been validated [34,35].

A recent study based on the Danish population registries explored the incidence of the full spectrum of mental disorders diagnosed at hospital before the age of 18 years [34]. The incidence of any disorder was 15.1% (CI 15.0-15.2) with a peak incidence of autism-spectrum disorders, attention-deficit/hyperactivity disorder (ADHD) and intellectual disability in early childhood. A higher incidence of these neuro-developmental disorders was seen in boys compared to girls, whereas anxiety disorder, depression and eating disorders was the most common among girls, and with a later onset [34].

The presentation, comorbidity and risk factors of mental disorders in 0-3-year-old children were explored in two register-based studies of nationwide population of 901,227 children followed until 36 months of age [35,36]. In the study by Koch et al. [35], we found that 1.8% of the child population was diagnosed at hospital with an ICD-10 mental disorder before the age of four years. Developmental disorders, including intellectual disability was overall the most frequent. Feeding and eating disorders and disorders of social functioning were the most frequent among the youngest children, 0-1 years, whereas pervasive developmental disorders,  including autism-spectrum disorders, and disorders of hyperactivity and inattention, ADHD, were increasingly diagnosed with age among the 0-3-year-olds. Comorbidity was found in 18% of the referred children, and in particular, high comorbidity was seen among neuro-developmental disorders, e.g intellectual disability, pervasive developmental disorders and ADHD [37], and  between neuro-developmental disorders and emotional and behavioural disorders. Further, high comorbidity was seen between ADHD and disorders of social functioning and behavioural disorders, and between pervasive developmental disorders and emotional disorders. Boy sex and being born small for gestational age were associated with an overall increased risk of mental disorders, and in particular developmental disorders, and so were maternal smoking in pregnancy, young maternal age and old paternal age [35].

Among 0-3 years old referred to hospital, disorders of feeding and eating (FED) were diagnosed with a cummulative incidence of 0.2% [36]. Premature birth, being born small for gestational age and with congenital malformations were associated with a three to nearly five fold increased risk of FED. While boys have a higher risk of overall mental disorders, and in particular for neuro-develomental disorders [35], girls showed a higher risk of FED. Also, being born in families of immigrant status and being the firstborn were associated with increased risk of FED in the first years of living [36].

Research on the developmental epidemiology of mental disorders in the first three years of living is still in its infancy. However, the research findings published so far highlight that early emerging psychopathology and impairing mental disorders at ages 0-3 years are as frequent as in older ages, showing patterns of comorbidity and risk factors that need to be addressed in clinical settings as well as in future research. More research in this field is needed, but so far, the current state of knowledge points to complex causal mechanisms of risks and early developmental trajectories involving both perinatal and psychosocial risk factors that have to be taken into account in multidisciplinary approaches in the clinical management of young children with mental disorders.