Does thumb-sucking run in families?

Genetic tie up

Also in this week's column:

Does thumb-sucking run in families?

Asked by Michael Woodhams of Palmerston North, New Zealand

Thumb-sucking is a "childhood body-focused behaviour". Others are nail-biting, scratching, hair-pulling, nose-picking, and a few others.

Babies are born with a sucking reflex. In fact, if this reflex is not present from birth, the infant cannot feed and quickly perishes. Babies have a natural urge to suck. They will suck on just about anything that touches their lips. The urge starts to decrease after the age of six months.

About 80 per cent of babies suck their thumbs. Most stop by themselves between the age of three and six years.

There has been no "thumb-sucking gene" found so far. However, recent Japanese research involving 1,131 pairs of twins found that there was a strong genetic influence in finger-sucking behaviour in 66 per cent of male twins and 50 per cent of female twins and in nail-biting behaviour in 50 per cent of both male and female twins.

The study is by Dr S Ooki of the Department of Health Science of Ishikwa Prefectural Nursing University in Kahoku and published in Twins Research and Human Genetics in August 2005.

The observation has long been made that thumb-sucking behaviour seems to run in families. Recently, there has been some research to back this notion. In an Austrian study examining families of children with eating disorders, it was found that children from certain types of families were statistically significantly more likely to exhibit thumb-sucking behaviour.

These families are what the researchers, Dr B.Mangweth and seven colleagues, call "body denying" families. They are families where parents are emotionally cold, disapproving, and show a lack of intimacy towards their children.

Dr Mangweth and colleagues are from the Department of Psychiatry at the Innsbruck University Hospitals. heir study appeared in Psychotherapy and psychosomatics in 2005.

Family factors and the experiences of children have a bearing on thumb-sucking, nail-biting, and a host of other forms of "oral habits". Indeed, a study of abnormal oral habits (for example, biting other people) in the children of war veterans in Iran showed that when children have brutalising experiences, they behave brutally with their bodies - including their mouths.

Dr Y Tassaei and two colleagues from the Shahid Sadoughi University of Medical Sciences and Health Services in Yazd, Iran write in the Journal of Clinical Pediatric Dentistry in 2005 that: "The prevalence rate [of abnormal oral behaviours] was highest in children, whose family members had been both crippled and freed prisoners of war, while the rate was lowest in children whose parents had been only prisoners of war without any lasting physical injury."

Interesting fact

Handedness is determined before you are born. The hand that is preferred when thumb-sucking as a fetus is almost certainly to be the preferred hand after birth. For example, all of 60 right-thumb sucking fetuses turned out to be right-handed in childhood. This was found in a study by Dr P G Hepper and two colleagues from the School of Psychology at Queen's University in Belfast, Northern Ireland, and published in Neuropsychologia in 2005.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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