Environmental rather than genetic factors determine the variation in the age of the infancy to childhood transition: a twins study

Heritability of Psychological Traits and Developmental Milestones in Infancy: A Systematic Review and Meta-analysis

IMPORTANCE

Although infancy is the most rapid period of postnatal growth and development, factors associated with variation in infant traits are not well understood.

OBJECTIVE

To synthesize the large twin study literature partitioning phenotypic variance in psychological traits and developmental milestones in infancy into estimates of heritability and shared and nonshared environment.

DATA SOURCES

PubMed, PsycINFO, and references of included publications were searched up to February 11, 2021.

STUDY SELECTION

Peer-reviewed publications using the classical twin design to study psychological traits and developmental milestones from birth to 2 years old were included.

DATA EXTRACTION AND SYNTHESIS

Data were extracted in line with Preferred Reporting Items for Systematic Reviews and Meta-Analyses and categorized using the International Classification of Functioning, Disability and Health: Children and Youth Version. Data were pooled in 3-level random effects models, incorporating within-cohort variance in outcome measurement and between-cohort variance. Data were analyzed from March 2021 through September 2021.

MAIN OUTCOMES AND MEASURES

The primary outcomes were monozygotic and dizygotic twin correlations. These were used to calculate genetic and shared and nonshared environment estimates.

RESULTS

Among 139 publications that were systematically retrieved, data were available on 79 044 twin pairs (31 053 monozygotic and 47 991 dizygotic pairs), 52 independent samples, and 21 countries. Meta-analyses were conducted on psychological traits and developmental milestones from 106 publications organized into 10 categories of functioning, disability, and health. Moderate to high genetic estimates for 8 categories were found, the highest of which was psychomotor functions (pooled h2, 0.59; 95% CI, 0.25-0.79; P < .001). Several categories of traits had substantial shared environment estimates, the highest being mental functions of language (pooled c2, 0.59; 95% CI, 0.24-0.86; P = .001). All examined categories of traits had moderate or high nonshared environment estimates, the highest of which were emotional functions (pooled e2, 0.42; 95% CI, 0.33-0.50; P < .001) and family relationships (pooled e2, 0.42; 95% CI, 0.30-0.55; P < .001).

CONCLUSIONS AND RELEVANCE

These findings may be an important source of information to guide future gene discovery research, public perspectives on nature and nurture, and clinical insights into the degree to which family history and environments may estimate major domains of infant functioning, disability, and health in psychological traits and developmental milestones.

Is the World Health Organization’s multicentre child growth standard an appropriate growth reference for assessing optimal growth of South African mixed-ancestry children?

In South Africa (SA), it has been estimated that one-third of boys and 25% of girls under the age of 5 years are stunted, according to the World Health Organization (WHO) Multicentre Growth Reference Study. During the past decade, research in developed and developing countries has shown that the international growth standard overestimates stunting and/or wasting when compared with population-specific growth references. Population-specific growth references typically incorporate genetic and environmental factors and can therefore better inform public health by identifying children who may be at risk for malnutrition, or who may be ill. Using the universal growth standard in SA may not be accurately assessing growth. In this article, environmental and genetic factors, and their influence on growth, are reviewed. These points are illustrated through a brief history of the peopling of SA, with an understanding of the socioeconomic and political climate – past and present. We discuss the uniqueness of certain population groups in SA, with contributions regarding some of the shortest peoples in the world and a history of sociopolitical inequities, which may mean that children from certain population groups who are perfectly healthy would underperform using the universal growth standard. Therefore, we suggest that a local population-specific growth reference would serve to better inform public health policies, and address childhood health equity and physical developmental pathways to adult health risk status.  

Uncoupling of the Infancy Life History Stage

BACKGROUND

The life history of Homo sapiens is unique in having a comparatively short stage of infancy which lasts for 2-3 years. Infancy is characterized by suckling of breast milk, the development of sensorimotor cognition, the acquisition of language, mini-puberty, deciduous dentition, and almost complete skull growth. Infancy ends with the infancy-childhood growth transition (ICT) and separation from the mother. In modern-day affluent societies, breastfeeding depends on the mother's decision and may happen at any age, and the characteristic traits of infancy have uncoupled. The data and theory for this contention are presented.

SUMMARY

The biological traits of mini-puberty and ICT characteristic of infancy occur before age 1 along with language acquisition. The cognitive (sensorimotor) component occurs by age 2, and the social component of separation from the mother by any age from 1 to 3 years. Key Messages: Human life history is based on a coherent stage of infancy which assumes coupling between the biological, cognitive, and social maturation of a baby. This is no longer the case in industrial societies and might never be so again. The upbringing of an infant needs to consider the new biology of this dissociated infancy and a new timetable of the infant's life-history events.

Family Size and the Age at Infancy-Childhood Transition Determine a Child's Compromised Growth in Large Families

BACKGROUND

Data on growth of Israeli school children show that children from Jewish ultra-orthodox Haredi and Bedouin Arab families have a higher prevalence of stature below the 3rd percentile. While these populations are usually from lower socioeconomic strata, they also have larger families. This study aimed to evaluate if family structure and the timing of a child's infancy-childhood transition (ICT) are central to variations in stature.

STUDY DESIGN

We analyzed the association between family size, birth order and inter-birth interval with child growth and the age at ICT in 3 groups of children, 148 high birth order children from large families (LF ≥ 6), 118 low birth order children from large families (LF ≤ 3) and 150 children from small families (SF).

RESULTS

High birth order children from large families were shorter in childhood than children from small families with a difference of 0.5 SDS in length. We found that birth length and birth order explained 35% of the total variance in infancy length whereas ICT age and infancy length explained 72% of the total variance in childhood length.

CONCLUSION

Infancy and childhood length are compromised in children from large families. As the family grows larger the younger children tend to be shorter. Reduced length gain in the period between infancy to childhood is when growth is most affected.

Timing of the Infancy-Childhood Growth Transition in Rural Gambia

The Karlberg model of human growth describes the infancy, childhood, and puberty (ICP) stages as continuous and overlapping, and defined by transitions driven by sequential additional effects of several endocrine factors that shape the growth trajectory and resultant adult size. Previous research has suggested that a delayed transition from the infancy to the childhood growth stage contributes to sub-optimal growth outcomes. A new method developed to analyze the structure of centile crossing in early life has emerged as a potential tool for identifying the infancy-childhood transition (ICT), through quantifying patterns of adjacent monthly weight-for-age z-score (WAZ) deviation correlations. Using this method, the infancy-childhood transition was identified as taking place at around 12 months of age in two cohorts of UK infants. Here, we apply this method to data collected as part of a longitudinal growth study in rural Gambia [the Hormonal and Epigenetic Regulators of Growth, or HERO-G study, N = 212 (F = 99, M = 113)], in order to identify the ICT and assess whether timing of this transition differs across groups based on sex or birth seasonality. We calculated Pearson correlation coefficients for adjacent monthly WAZ score deviations. Based on the patterns of change in the correlation structure over time, our results suggest that the infancy-childhood transition occurs at around 9 months of age in rural Gambian infants. This points to an accelerated ICT compared to UK infants, rather than a delayed ICT. A comparatively later transition, seen in UK infants, allows maximal extension of the high rates of growth during the infancy stage; an earlier transition as seen in Gambian infants cuts short this period of rapid growth, potentially impacting on growth outcomes in childhood while diverting energy into other processes critical to responses to acute infectious challenges. Growth in later developmental stages in this population offers an extended window for catch-up.

Sexual Dimorphism of Size Ontogeny and Life History

Background: Ecological and physiological factors and social and economic constraints affect sex-specific body size. Here, we used the male/female (M/F) height ratio as an indicator of the combined effect of genetic and sex characteristics. We hypothesized that (1) sexual dimorphism in body size will be established during infancy and adolescence when growth velocity is maximal, (2) living standards and health are important factors which can affect sexual dimorphism in body size, (3) variations in sexual dimorphism in body size are due to the differential response of boys and girls to environmental cues, and (4) sexual dimorphism in body size will be more pronounced in those populations whose average height and weight are the greatest. Methods: To study the ontogeny of sexual dimorphism from birth until the age of 18 years, we used the 2000 CDC growth data. Data on height by country, life expectancy, and gross domestic product (GDP) per capita based on purchasing power parity were extracted from the national accounts data of NCD Risk Factor Collaboration, the World Bank, Eurostat: Demographic Statistics, Secretariat of the Pacific Community: Statistics and Demography Program, and the US Census Bureau. Results: We found that sexual dimorphism in body size starts at age 1 month, peaks at age 3 months, and diminishes by age 24 months. During childhood, there is no sexual difference in body size, and it is gradually established when the boys enter puberty. The M/F height ratio correlates positively with the average male and female height and weight by country. Conclusion: Sexual dimorphism in body size occurs when (a) the growth velocity is maximal during infancy and adolescence, (b) living standards are high, and health correlate positively with male/female height ratio. Anthropological studies and our results emphasize mostly the female resiliency hypothesis: shorter male heights in times of environmental stress lead to smaller sexual dimorphism in body size.

Does idiopathic congenital talipes equinovarus have an impact on attainment of developmental milestones? A multicentre international study

PURPOSE

The Ponseti method is a well-established approach to treating clubfoot. Potentially, both the underlying pathology and adherence to post-correction bracing can affect lower limb function and age of independent standing and walking. This cohort study investigates the age at which infants with idiopathic clubfoot treated using the Ponseti method achieved three selected developmental milestones and whether or not this correlated with treatment compliance.

METHODS

A prospectively collected database from four centres was visited. Inclusion criteria were patients with idiopathic clubfoot with no comorbidities or prior treatment. Age at attainment of independent standing, walking, nocturnal continence was compared across three groups: I) congenital talipes equinovarus (CTEV) children compliant with treatment; II) CTEV children non-compliant with treatment; and III) typically-developed siblings. Minimum follow-up was five years.

RESULTS

In all, 130 patients (198 feet) fitted the inclusion criteria: 43:87 (F:M). Standing was achieved by a mean 12.0 months in group I (sd 2.50); 12.0 months (sd 2.0) in II and ten months (sd 3.0) in III. Walking was achieved by a mean 15 months (sd 4.0) in group I, 14 months (sd 1.75) in II and 12 months (sd 3) in III, respectively. Both the compliant and non-compliant CTEV children were significantly slower at achieving standing and walking compared to sibling controls (p < 0.0001). There was no significant difference between age of nocturnal continence between the three groups.

CONCLUSION

Infants with idiopathic clubfoot treated according to the Ponseti method achieve independent standing and walking approximately two months later than their typically-developed siblings. The delay is not related to the use of the foot abduction brace.

LEVEL OF EVIDENCE

III.

Outcomes of pubertal development in girls as a function of pubertal onset age

BACKGROUND

The relationship between pubertal onset and tempo and pubertal growth is controversial. We hypothesized that the age at onset of girls' puberty predicts pubertal tempo and the rate of pubertal progression.

METHODS

We analysed the data of 380 girls from the prospective Study of Early Child Care and Youth Development (SECCYD), who were recruited in the USA from 1991-2006 and followed from birth to age 15.5 years. We used the following indicators: thelarche age (Tanner stage B2), pubarche age (P2), menarche age (M), the age when breast (B5) and pubic hair (P5) became fully mature, pubertal growth, pubertal duration (time from B2 to B5), pubertal progression (time from B2 to M). We clustered the girls according to B2 age into early onset (EO;<9.4 years), intermediate (IO;9.4-10.5 years), late onset (LO;>10.5 years).

RESULTS

All indicators of pubertal onset and conclusion occurred earlier in the EOs than in the LO; yet, the differences in the age at main pubertal milestones lessened as puberty progressed: two years for B2; -1.4 years for M; - one year for B5. In EO, puberty was one year (average) longer than in LO. Although EO grew 7 cm (average) more than LO, their heights at B5 were comparable. There was a significant relationship between the thelarche age and puberty tempo (r=0.23, P<0.0001).

CONCLUSIONS

The study highlights the predictive nature of variation in the onset age of puberty on its progression and duration. These results are reassuring in this context and will add to clinicians' and parental understanding of the expected milestones of puberty.

Weight centile crossing in infancy: correlations between successive months show evidence of growth feedback and an infant-child growth transition

BACKGROUND

Early rapid weight gain is associated with later overweight, which implies that weight centile crossing tracks over time.

OBJECTIVE

Centile crossing is defined in terms of the change or deviation in weight z score during 1 mo, and the correlations between successive deviations are explored at different ages.

DESIGN

Two Cambridge (United Kingdom) growth cohorts were used: Widdowson (1094 infants born during 1959-1965) and the Cambridge Infant Growth Study (CIGS; 255 infants born during 1984-1987), each with weights measured monthly in the first year. Weights were converted to WHO age- and sex-adjusted z scores, deviations were calculated as the change in z score between adjacent measurement occasions, and the correlations between deviations were studied.

RESULTS

In both cohorts, the correlations between successive monthly deviations were positive in the first 6 mo and highest at ages 3-4 mo (r = 0.3, P < 0.0001), whereas after 6 mo they were negative and were lowest at ages 10-11 mo (r = -0.3, P < 0.0001), with the correlation decreasing linearly with age between these extremes. Thus, during the first 6 mo of age, infants crossing centiles in 1 mo tended to continue crossing centiles in the same direction the following month, whereas after 6 mo they tended to cross back again. This represents positive and negative feedback, respectively. At age 12 mo, the correlation was close to zero, which suggests an infant-child transition in growth.

CONCLUSIONS

The results confirm that weight centile crossing tracks over time, with the correlations between successive periods that change with age suggesting a complex feedback mechanism underlying infant growth. This may throw light on the link between early rapid weight gain and later overweight. Clinically, the correlations indicate that when predicting future weight from current weight, recent centile crossing affects the prediction in an age-dependent manner.