Twin Study Finds Genetics Play a Major Role in Infant Crying Duration

Recent research from Uppsala University and the Karolinska Institutet has shed light on the genetic influence on infant crying behavior. The study, published in JCPP Advances, analyzed responses from parents of 1,000 twins in Sweden at two and five months of age to understand how genetics and environment impact crying and settling routines during early infancy. The key finding revealed that genetics significantly determine how much infants cry daily—about 50% at two months and increasing to 70% by five months—highlighting that parental influence on crying may be limited.

The research involved comparing identical (monozygotic) twins, who share nearly all their genes, with fraternal (dizygotic) twins, who share about half. This comparison allowed scientists to estimate the genetic contribution to crying behaviors. The results indicated that genetics had a larger role in crying duration than in sleep patterns, such as the number of night awakenings, which were more influenced by environmental factors like sleep routines.

Interestingly, the study also explored how quickly infants fall asleep after being put to bed, noting that this aspect is initially mainly determined by environmental influences at around two months but becomes more genetically influenced by five months. These findings suggest that parents’ efforts to help babies settle might be most effective during the first months of life.

The twin study utilized data collected longitudinally until children reached 36 months, providing insights into how sleep and crying behaviors evolve over time. The research emphasizes the importance of genetics in early behavioral traits, offering reassurance to parents that crying is largely inherent and not solely influenced by caregiving practices. Future research may focus on identifying specific environmental factors that can modify sleep routines and behaviors in infants.

Source: [https://medicalxpress.com/news/2025-07-twin-reveals-genetics-largely-infants.html]