Summary: A new twin study reveals that how much an infant cries is strongly influenced by genetics, leaving parents with limited control over this behavior. Researchers found that at two months, genetics accounted for around 50% of crying duration, rising to 70% by five months.
In contrast, night awakenings and time to settle were more shaped by environmental factors, such as sleep routines, particularly early on. These findings highlight the role of both nature and nurture in infants’ sleep and crying behaviors, guiding future research on effective interventions.
Key Facts:
- Genetic Influence: Up to 70% of crying duration by 5 months is genetically determined.
- Environment Matters: Night awakenings and settling time are more influenced by sleep environment and routines.
- Twin Study Insight: Comparing identical and fraternal twins reveals the genetic versus environmental contributions.
Source: Uppsala University
How much an infant cries is largely steered by their genetics and there is probably not much that parents can do about it.
This has been shown in a new Swedish twin study from Uppsala University and Karolinska Institutet in which researchers investigated how genetics and environment influence infants’ crying duration, sleep quality and ability to settle during the first months of life.
The study, which was recently published in JCPP Advances, is based on questionnaire responses from parents of 1,000 twins spread across Sweden. The parents were asked questions about their children’s sleep, crying and ability to settle when the twins were 2 months old and then again at 5 months old.
The researchers were interested in finding out how genetics and environment influence these behaviours during the first months of life – something that no study has done before.
The clearest results were seen when the researchers analysed how long the children cried per day.
“What we found was that crying is largely genetically determined. At the age of 2 months, the children’s genetics explain about 50 per cent of how much they cry. At five months of age, genetics explain up to 70% of the variation.
“For parents, it may be a comfort to know that their child’s crying is largely explained by genetics, and that they themselves have limited options to influence how much their child cries,” says Charlotte Viktorsson, postdoctoral fellow in psychology and lead author of the study.
The remaining percentage that cannot be explained by genetics was explained by what the researchers call ‘unique environment’ – factors in the children’s environment or life situation that are unique to each child and cannot be identified precisely from the questionnaire responses.
Twin studies reveal the importance of genetics
The participants were recruited by letter, which was sent to families with twins aged 1–2 months. These families were identified from the population register. To be able to capture how much of a behaviour is genetically determined, the researchers compared identical (monozygotic) twins with fraternal (dizygotic) twins.
The advantage of studying twins is that they share important factors such as home environment, family situation, and socio-economic status.
If identical twins become more similar to each other than fraternal twins in terms of a certain trait, such as how much they cry, it is seen as an expression of the importance of genetics for that trait.
The environment plays a role in infants’ time until settled
Using the same method, the researchers also analysed the number of times the children woke up at night. Here, genetics played less of a role. The number of awakenings during the night was mainly influenced by environmental factors, which can include sleep routines and the environment in which the child sleeps
In the questionnaire, the parents were also asked to state how long it took from the child being put to bed until they were asleep.
“How rapidly the infant settles was primarily due to the environment at 2 months of age, but by 5 months, their genetics had gained some significance. This reflects the rapid development that occurs in infants, and may indicate that parents’ efforts in getting their child to settle may have the greatest impact in the first months,” says Charlotte Viktorsson, who led the study.
However, it is difficult to draw conclusions about which interventions are effective based on this type of observational study.
“Although we cannot see which specific environmental factors influence the number of awakenings during the night, or how long it takes until the child settles, this study points out a direction for future studies with a focus on sleep routines,” she says.
The researchers have followed the twins up to 36 months of age, allowing them to see how sleep and crying change as the children get older. The current study is thus the first in order based on this data.
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.
“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. JCPP Advances
Abstract
Genetic and environmental influences on sleep quality, ability to settle, and crying duration in 2- and 5-month-old infants: A longitudinal twin study
Background
Sleep and behavioral regulation are both vital for early healthy development. Yet, little is known about the relative contribution of genetic and environmental factors to early sleep and regulatory behaviors, or how these etiological influences may change during the first months of life.
Methods
Genetic and environmental influences on sleep, settle, and crying behaviors at 2 and 5 months were examined in 998 twins, using a classical twin design. In addition, polygenic scores were derived for a range of sleep behaviors, as well as psychiatric and neurodevelopmental conditions.
Results
Genetic influences (A) explained a large part of the variation in duration of crying at both 2 and 5 months (A = 0.29–0.70) and in settle ability at 5 months (A = 0.51–0.67). Shared environment (C) primarily influenced number of wakeups per night at both ages (C = 0.61–0.90) and settle ability at 2 months (C = 0.36–0.65). Longitudinal analyses suggested modest shared genetic influence on settle ability in the daytime across the ages (24%), and non-significant shared genetic estimates for ability to settle in the evening and at nighttime.
There was moderate shared influence of shared environmental factors on number of wakeups per night (56%) and modest but significant shared genetic influence on crying duration in the evening and nighttime (17%–33%). Unique environmental effects were mostly specific to each age. Finally, autism polygenic score associated with longer crying duration in the evening at 2 months (β = 0.16, p = .002).
Conclusions
Etiological influences tended to change from 2 to 5 months, reflecting a highly plastic period in infant brain development and in child-environment interactions.
