The IQs of 2- to 4-year-olds who received regular spankings from their parents dropped by more than 5 points over four years, compared with kids who were not spanked.
"The practical side of this is that paediatricians and child psychologists need to start doing what none of them do now, and say, 'Never spank under any circumstances,'" says Murray Straus, a sociologist at the University of New Hampshire in Durham, who led the new study along with Mallie Paschall at the Prevention Research Center in Berkeley, California.
Theirs isn't the first evidence that spanking children comes with a cost: several previous studies have hinted at the association, and a recent brain-imaging study found that children who underwent severe corporal punishment had less brain grey matter – which includes neurons – compared with other children. Stress, anxiety and fear might explain why spanking slows cognitive development.
However, the new research makes a stronger case for a cause-effect relationship between spanking and intelligence than other studies, says Elizabeth Gershoff, a child development researcher at the University of Texas, Austin, who was not involved in the new study. This is because it examined children over the course of four years and accounted for many confounding variables, such as a parent's ethnicity, education and whether they read to their children or not.
Straus and Paschall analysed data collected in the 1980s as part of a nationwide survey of children's health. In 1986 a previous study measured the IQ of 1510 2- to 9-year olds and also noted how often their mothers meted out corporal punishment. The researchers repeated the IQ tests four years later.
Straus and Paschall split the kids into two age groups – 2 to 4 years and 5 to 9 – because some child psychologists contend that occasional spanking is acceptable in toddlers, but not older children.
The figures revealed that 93 per cent of mothers had spanked 2- to 4-year-olds once or more per week, whereas only 58 per cent had turned to physical discipline with the older kids. Nearly half of toddlers' moms had spanked their children three or more times per week, Straus and Paschall found.
Four years later, younger children who had never felt their mother's palm had gained an average of 5.5 IQ points compared with kids who had suffered corporal punishment, while older unspanked children had gained 2 IQ points, on average.
These results cast doubt on the practice of spanking only younger kids, Straus says. "It's one of the cruel ironies that younger children are more at risk because their brain is in the most formative part of development."
Although Straus and Paschall accounted for many other factors that could affect intelligence before concluding that corporal punishment leads to lower IQ, spanking isn't a guarantee of intellectual mediocrity.
In the younger children, the thing that made the biggest difference to IQ scores was whether or not mothers provided cognitive stimulation. This was more important than anything else, including corporal punishment.
"Let's say you have a child that comes from educated parents, warm and supportive parents, who provide cognitive stimulation, and they spank: that kid's going to be OK anyhow, [though] maybe not quite as OK [as the child would be] if they didn't spank," Straus says.
However, he has little patience for the argument that spanking accomplishes what other forms of discipline cannot. "The research simply doesn't show that," he says. "Spanking doesn't work any better with a 2-year-old than saying, 'No, don't do that.'"