Summary: A new Swedish twin study shows that how long an infant cries is strongly influenced by genetics, with heritability estimates rising from roughly 50% at two months to about 70% by five months. The study also finds that night awakenings and how quickly infants settle are more affected by environmental factors such as sleep routines, especially early in life.
Researchers from Uppsala University and Karolinska Institutet analyzed parent questionnaires from nearly 1,000 twins across Sweden to separate genetic and environmental contributions to early crying and sleep behaviors. The results clarify how nature and nurture interact during the first months of life and point to where future intervention studies should focus.
Key facts
- Genetic influence: Crying duration is highly heritable, accounting for about 50% of variation at 2 months and up to 70% at 5 months.
- Environmental influence: Night awakenings and time to settle are more strongly shaped by shared and unique environmental factors such as sleep routines and the sleep environment.
- Twin design: Comparing identical (monozygotic) and fraternal (dizygotic) twins enabled the researchers to estimate genetic versus environmental contributions.
Source: Uppsala University
What the study examined
This longitudinal twin study collected questionnaire data when the infants were about 2 months old and again at 5 months. Parents reported on crying duration, the number of night awakenings, and how long it took the infant to settle after being put to bed. By comparing similarities between identical and fraternal twins, the researchers estimated the relative contributions of genetics, shared environment (factors common to both twins), and unique environment (factors specific to each child).
Main findings
The clearest and most striking result was for crying duration. Genetics explained about half of the variation in crying at two months and increased to approximately 70% by five months. For parents, this suggests that much of individual differences in how much an infant cries are inherited and therefore relatively resistant to change through parenting strategies alone.
In contrast, the number of times infants woke during the night was largely explained by shared environmental factors. This includes elements such as household routines, sleep arrangements, and other aspects of the shared home environment. Time to settle the infant after being put to bed showed a stronger environmental influence at two months, but genetic contributions became more apparent by five months — reflecting rapid developmental shifts during this period.
Why twin studies matter
Twin studies are a proven method for disentangling heredity from environment because identical twins share nearly all their genes while fraternal twins share, on average, half. When identical twins are notably more similar on a trait than fraternal twins, researchers infer a genetic contribution. In this study, that pattern was clear for crying duration and emerged over the early months for the ability to settle.
Implications for parents and research
For caregivers, the findings may provide reassurance that some crying patterns reflect biological differences rather than parenting failings. At the same time, the results highlight that environmental factors—particularly those shaping night awakenings and early settling—are meaningful and modifiable. The study authors note that observational data cannot prove which specific interventions work best, but the results direct future research toward testing sleep routines and other environmental strategies during early infancy.
Additional details from the study
The analysis of 998 twins used a classical twin design and also explored polygenic scores for sleep-related traits and neurodevelopmental conditions. Genetic influences explained a substantial portion of crying duration at both ages and settle ability at five months, while shared environment predominantly drove night wakings. The researchers also reported a modest association between an autism polygenic score and longer evening crying at two months.
About this genetics research news
Author: Charlotte Viktorsson
Source: Uppsala University
Contact: Charlotte Viktorsson – Uppsala University
Image: The image is credited to Neuroscience News
Original research: Open access. Title: “Genetic and environmental influences on sleep quality, ability to settle, and crying duration in 2- and 5-month-old infants: A longitudinal twin study” by Charlotte Viktorsson et al., published in JCPP Advances.
Abstract (summary)
Background
Sleep and behavioral regulation are crucial for early development. This study examined how genetic and environmental influences contribute to sleep quality, the ability to settle, and crying duration in the first months of life, and whether these influences change between two and five months of age.
Methods
Using a classical twin design, the study analyzed data from 998 twins at 2 and 5 months and computed polygenic scores for sleep traits and neurodevelopmental and psychiatric conditions.
Results
Genetic factors explained a large portion of variation in crying duration at both ages (estimates ranging roughly from 29% to 70%) and in settling ability at five months (about 51%–67%). Shared environmental factors mainly influenced the number of night awakenings (about 61%–90%) and settling ability at two months (about 36%–65%). Unique environmental effects were mostly age-specific. An autism polygenic score was modestly associated with longer evening crying at two months.
Conclusions
Etiological influences shift between two and five months, reflecting a highly plastic developmental window in which both genetic predispositions and environmental conditions interact to shape early sleep and regulatory behavior.