Genetics Play Major Role in How Much Babies Cry
Infant crying is largely driven by genetics, with environmental factors playing a smaller role in early development.
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A study from Uppsala University and Karolinska Institutet has found that genetic factors largely determine how much an infant cries. Drawing on data from 1,000 twin pairs across Sweden, the research used parental questionnaires to assess sleep and crying behaviours when the children were 2 and 5 months old. It is one of the first studies to isolate genetic and environmental influences on infant sleep and crying during early development.
Crying duration linked strongly to inherited traits
The researchers found that crying duration showed the highest genetic influence among the behaviours studied. At 2 months of age, genetics accounted for approximately 50% of the variation in crying time. By 5 months, this influence increased to about 70%. The remaining variance was attributed to ‘unique environmental factors’, elements of a child’s life not shared with their twin or not easily identifiable through the study’s methods.
"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.”
Dr. Charlotte Viktorsson.
Genetic analysis supported by twin comparisons
To disentangle inherited traits from environmental ones, the team compared monozygotic (identical) twins, who share nearly all their DNA, with dizygotic (fraternal) twins, who share roughly half. Both types of twins share home and family environments. Greater behavioural similarity among identical twins indicates a stronger genetic component. This approach allowed the researchers to assess the heritability of crying, sleep quality and ease of settling.
Monozygotic twins
Also known as identical twins, these twins originate from a single fertilised egg that splits and develops into two embryos.Dizygotic twins
Also known as fraternal twins, these twins result from two separate eggs fertilised by two different sperm cells.Environmental effects more evident in sleep behaviours
Not all behaviours were as strongly influenced by genetics. Night awakenings, for example, were primarily shaped by environmental factors. These might include bedtime routines, physical sleep environment or caregiving practices, although the study design did not allow for the identification of specific causes.
Ease of settling showed a mixed influence. At two months of age, this behaviour was mainly environmental, but by five months, genetic contributions became more pronounced. The developmental trajectory suggests that while early interventions might influence sleep behaviour, some traits become more genetically embedded as infants grow.
Longitudinal data may provide further insights
This publication is the first from a broader dataset that will track the same twins until they are three years old. Future analyses may clarify how the roles of genetics and environment shift over time and how these early traits relate to later cognitive or emotional development.
Reference: Viktorsson C, Yahia A, Taylor MJ, Ronald A, Tammimies K, Falck-Ytter T. Genetic and environmental influences on sleep quality, ability to settle, and crying duration in 2- and 5-month-old infants: A longitudinal twin study. JCPP Advances. 2025;n/a(n/a):e70023. doi: 10.1002/jcv2.70023
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