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.


Facts about the study
The parents who participated in the study were asked how long their children cried, how often they woke up at night, and how long it took until they settled. There was large individual variation between the children. For example, some children could wake up a total of up to 10 times per night. The figures below show averages.
2 months:
Crying duration (per 24 hours): about 72 minutes
Wakeups: 2.2 times per night
Time until settled: about 20 minutes

5 months:
Crying duration (per 24 hours): about 47 minutes
Wakeups: 2.1 times per night
Time until settled: about 14 minutes