Early life emotional trauma may stunt intellectual development, indicates the first long term study of its kind, published online in the Journal of Epidemiology and Community Health.
The impact seems to be the most damaging during the first two years of a child's life, the findings suggest.
The US researchers tracked the development of 206 children from birth to the age of eight years, who were taking part in the Minnesota Longitudinal Study of Parents and Children. This study, which started in 1975, looks at which factors influence individual development.
Every few months they assessed the participating families, using a mix of observing mother-child interactions at home and in the laboratory, interviews with the mother, and reviews of medical and child protection records.
From these data, they rated whether a child was abused physically, sexually or emotionally; endured neglect; or witnessed partner violence against his/her mother at specific time points up to the age of 5+ years.
The children's intellectual development was then assessed using validated scales at the ages of two years, 5+ years, and 8 years, and exposure to maltreatment or violence was categorised according to whether these occurred during infancy (0-24 months) or pre-school (24-64 months).
Around one in three of the children (36.5%) had been maltreated and/or witnessed violence against his/her mother by age 5+.
In just under one in 20 (4.8%) this occurred in infancy; in 13% this was during the pre-school period; and in around one in five (18.7%) this occurred during both periods.
Analysis of the data showed that children who had been exposed to maltreatment and/or violence against the mother had lower scores on the cognitive measures at all time points.
The results held true even after taking account of factors likely to influence IQ development, such as social and economic factors, mother's IQ, weight at birth, birth complications, quality of intellectual stimulation at home, and gender.
The effects were most noticeable for those children who had experienced this type of trauma during the first two years of their lives, the findings showed.
Their scores were an average of 7.25 points lower than those of children without early exposure, even after accounting for other risk factors.
"The results suggest that [maltreatment and witnessing domestic violence] in early childhood, particularly during the first two years, has significant and enduring effects on cognitive development, even after adjusting for [other risk factors]," write the authors.
They go on to say that their findings echo those of other researchers who have identified changes in brain circuitry and structure associated with trauma and adversity in early life.
The early years of a child's life are when the brain is developing most rapidly, they say, adding, "Because early brain organisation frames later neurological development, changes in early development may have lifelong consequences."
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