Researchers find major flaws in the historical clinical trials used to justify spanking

A recent study suggests that there is no experimental evidence proving physical punishment is an effective way to discipline children. The findings indicate that alternative, non-physical strategies are just as effective at encouraging child cooperation, without the potential risks associated with spanking. The research was published in the journal Child Abuse & Neglect.

Spanking remains a widely practiced form of discipline, considered normative and socially acceptable in many cultures. Despite disapproval from some health organizations, it is estimated that globally, two out of every three children aged two to four years have been spanked.

Scientists conducted the new study in response to ongoing debates about the merits of physical punishment. Recently, some academics published a commentary arguing that strict experimental trials provide evidence that spanking is an effective way to enforce child compliance. These proponents claimed that laws banning physical discipline are misguided and that spanking should remain an available option for parents.

“We explored this topic because a recent invited commentary in a psychiatry journal advocated use of spanking as a means of enforcing child compliance. The authors argued that the ‘most rigorous’ clinical trials (specifically, [randomized controlled trials]) validate the effectiveness of spanking,” said Leslie Atkinson of Toronto Metropolitan University, the corresponding author of the new research.

“They argued further that studies showing a positive link between spanking and developmental difficulties (e.g., child behavioural and emotional problems) are not designed to assess causality (e.g., child behavioural difficulties could lead to more spanking, rather than vice versa). They conclude that spanking is an effective disciplinary strategy.”

“We were surprised at this perspective, given that the UN Convention on the Rights of the Child requires that children be protected from physical punishment. At present, only 70 countries are signatory to the Convention.”

The debate is highly relevant to current legal battles in North America. The United States is not a signatory to the Convention and does not legally ban corporal punishment at the state or national level. In Canada, the Supreme Court previously upheld the use of reasonable physical force for correction, but this ruling is currently being challenged in Parliament.

The scientists wanted to assess the scientific foundation of the claims supporting these laws. They focused their attention on four specific clinical trials published between 1981 and 1990. Proponents of physical discipline frequently cite these four trials as proof that spanking works.

The researchers sought to determine if these older studies actually hold up to modern scientific standards. They also wanted to evaluate several combined analyses of these studies, known as meta-analyses. A meta-analysis is a statistical technique that combines the results of multiple independent studies to find an overall trend.

Past meta-analyses have historically influenced court decisions and professional pediatric guidelines. However, the scientists identified statistical problems within some past meta-analyses that favored spanking. Many of those previous reviews only examined one or two narrow measures of child compliance, ignoring a wide variety of other behavioral outcomes recorded in the original trials.

Some earlier reviews also used mathematical techniques that tend to artificially inflate the apparent success of an intervention. For example, instead of comparing the final behavior of the spanked group directly against the non-spanked group, they measured behavior changes from before the discipline to after. This statistical choice breaks the rules of randomized experiments and exaggerates the reported benefits.

Their goal was to objectively evaluate whether these four older trials genuinely support the claim that spanking is better than non-physical discipline. To evaluate the evidence, the researchers first conducted a detailed review of the four historical trials. These trials originally tested spanking as a backup method for enforcing timeouts among oppositional children aged two to six years old.

In these older experiments, a mother would place her child in a chair for a timeout. If the child tried to leave the chair, the mother would use different assigned strategies. These strategies included spanking the child, returning the child to the chair while holding them, or placing the child in a room blocked by a plywood barrier.

Other conditions allowed the child to leave the chair without penalty or involved no consequences at all. The sample sizes in these original experiments were very small. They included only four, eight, or nine mother and child pairs per experimental condition.

The researchers evaluated the design of these trials using a standardized tool designed to detect bias in scientific studies. They found that three of the four trials had a high risk of bias due to flawed procedures by modern standards. For example, the ways children were assigned to different disciplinary groups were not rigorously randomized.

When examining the oldest trial from 1981, the researchers noticed a major design flaw that past reviews had overlooked. The children in the spanking group ended up spending significantly more time in the timeout chair than the children in the non-physical group. This means that any increase in child cooperation might have been caused by the longer timeout duration, rather than the spanking itself.

The researchers also determined that the original trials lacked external validity. External validity refers to how well the results of a study apply to real-world situations and diverse populations outside of a laboratory setting. Because the original trials only involved mothers in highly controlled clinical settings over thirty years ago, their findings do not easily apply to modern families.

In addition to the outdated clinical settings, the original experiments only tested children on specific types of instructions, such as telling a child to pick up a toy. They never tested instructions that required a child to stop a bad behavior. Research provides evidence that a child’s willingness to follow a direct command does not always predict their willingness to stop a prohibited action.

Attitudes toward parenting have also shifted massively since the 1980s. Modern studies suggest that the psychological impact of physical discipline is heavily influenced by how common and accepted the practice is within a specific community. Because the original trials took place in a single narrow context decades ago, they cannot account for these broader cultural shifts.

After reviewing the individual trials, the scientists conducted their own updated meta-analysis. This new analysis combined data from the four trials, encompassing a total of sixty-eight mother and child pairs.

The scientists compared the effectiveness of spanking against all the other non-physical strategies combined. They calculated effect sizes, which are numbers that represent the strength of a relationship between two variables. To account for the extremely small sample sizes in the older studies, the scientists used a specialized statistical metric called Hedges’ g, which is designed to prevent exaggerated results.

They also calculated confidence intervals to estimate the range in which the true effect of spanking likely falls. The resulting ranges were extremely wide, which indicates a very high level of uncertainty about the actual effectiveness of physical discipline. Ultimately, the researchers found no significant difference in effectiveness between spanking and the combined alternative strategies.

When they specifically compared spanking to the strategy of allowing the child to leave the timeout early, the difference in compliance was not statistically significant. This finding was based on data from two of the trials involving thirty-four mother and child pairs. Similarly, spanking was not significantly more effective than using a physical barrier to enforce the timeout.

In fact, the data provided evidence that the barrier method tended to be slightly more effective, though not by a significant margin. This comparison relied on data from three trials involving thirty-four pairs. The scientists also compared spanking to physically holding the child in the timeout chair.

Based on data from one trial of eighteen pairs, spanking did not prove to be more effective than holding. Finally, they found that spanking actually resulted in more disruptive behavior during timeouts compared to simply placing the child in a room. Together, these statistics show that spanking does not reliably outperform non-physical discipline.

“We decided to assess the literature on which the claims regarding the effectiveness of spanking were based,” Atkinson told PsyPost. “The literature consists of four controlled trials published between 1981 and 1990, and a meta-analysis of these trials published in 2024. We reviewed the trials one by one, and found that 1) they are poorly designed by today’s standards, with high risk of bias, 2) they offer ambiguous support for spanking at best, and 3) such support as exists pertains to very constricted circumstances. In addition, 4) the results were not generalizable to today’s population.”

These new findings diverge from some recent publications that advocate for limited spanking. For instance, a recent review of long-term observation studies suggested that occasional, mild spanking had minimal negative effects on behavior. That particular study argued that spanking could be an effective backup tool for parents of young children when milder responses fail.

Another previous analysis by scientists at Purdue University also suggested that infrequent hand spanking might decrease defiant behaviors. Those researchers looked at annual data from children aged six to eight in Tennessee and Indiana. They argued that previous studies exaggerated the harms of spanking by failing to separate individual personality differences from the effects of the discipline itself.

At the same time, the new findings align with a broad body of global research highlighting the potential risks of physical discipline. For example, a recent major study examining children in Bhutan, Cambodia, Ethiopia, and Rwanda found that spanking was negatively associated with cognitive and emotional development. That international study showed that children who were spanked had poorer outcomes in numeracy, literacy, and emotional regulation.

The debate over the effectiveness of spanking is hampered by the extremely small number of experimental findings. Because modern ethical guidelines prevent scientists from conducting new experiments that involve spanking children, the available experimental data will likely always remain sparse. This lack of data undermines the certainty of any broad statistical conclusions. The small sample sizes mean that mathematical estimates of effectiveness are relatively unstable.

Still, the researchers emphasize that this lack of evidence is exactly the point. People cannot claim that experimental evidence proves spanking works when the existing experiments are this limited and flawed.

“Our meta-analysis was based on the only four spanking trials available (and likely that ever will be available due to ethical considerations),” Atkinson said. “This paucity undermines the certainty of our meta-analytic results. However, it does show that there is currently no experimental evidence supporting the effectiveness of spanking, contradicting claims to the contrary.”

“Other literature shows that physical punishment is linked to negative developmental outcomes, although the causal nature of these findings is indeterminate. Together, these literatures suggest that the most prudent parenting strategies involve positive, relational parenting (e.g., positive reinforcement, praise in particular, and natural/logical consequences) that have been proven effective.”

“At this point, it would be fair to say that the use of physical punishment is a high-risk parenting strategy,” Atkinson concluded.

The study, “Is there experimental evidence for the effectiveness of disciplinary spanking? A review and Meta-analysis of four controlled trials,” was authored by Leslie Atkinson, Jennifer Khoury, Meghan Kenny, and Andrea Gonzalez.