Jeromy Anglim's Blog: Psychology and Statistics

Friday, September 25, 2009

Spanking Lowers Child IQ | Correlation is not Causation

As I was waiting for a coffee this morning, I had a cursory glance at the paper and found The Herald Sun reporting on a study stating that:
"A SPANK on the bottom, long used by parents to discipline a naughty child, could cause more than tears... It's now thought the age-old disciplinary method may also lower a child's IQ, with those spanked up to three times a week having a lower IQ due to psychological stress."

I then said to myself,  "If these researchers were able to determine that spanking has a causal effect, I guess they must have done an experimental study?" But how did they manage to get that past ethics? The hypothetical experiment that would allow for an unambiguous causal inference on the effect of spanking would involve assigning one-group of children to a no-spanking group and one group to a "spanking up to three times a week group". The mean IQ of the two groups could then be compared. And a logical inference could be drawn about he mean causal effect of regular spanking.
I read on:
"After studying 800 toddlers aged between two and four for four years, [the researcher] found those who were subjected to spanking had an IQ five points lower than that of a child who wasn't physically disciplined."
It becomes clearer. It's an observational study, albeit longitudinal. Thus, the research shows an association between child IQ and spanking. Last I checked, an association in an observational study does not mean causation. However, an association is often one piece of evidence in an argument about a causal effect.

To make the leap from an association to a causal inference it is useful to show that there are no alternative causal explanations, or at least argue that the alternatives are less plausible. In the presently discussed research on spanking, let's look at a few alternative causal explanations:

  1. Less intelligent parents have less intelligent children and less intelligent parents are more likely to smack their children.
  2. Less intelligent children behave in ways that provoke smacking responses in parents more often.
  3. Low socio-economic status and a generally poor parenting environment leads to both a greater likelihood of smacking and reduces intelligence in the child.
The above three alternative causal models assume that any association between child smacking and intelligence is, or is largely, epiphenomena. I find all of them more plausible than the claim presented in the newspaper. There is strong evidence for the genetic component of intelligence. And there are no doubt associations between socio-economic status and quality of parenting. I'm no expert on this literature, but there's a general principle at work:
To cause a big Dependent Variable to change, you need another big Independent Variable.
What do I mean by this?
Intelligence is famous for being difficult to modify. It has a strong genetic component and major interventions have often struggled to make appreciable changes. It's a big variable in that it represents the overall cognitive functioning or capacity of an individual. However, without wanting to weigh into debates about when smacking is or is not detrimental to a child's development, child abuse seems like the big variable, whereas smacking in and of itself seems like only a moderate variable. Thus, prima facie I am suspicious of the causal inference.

Thus, the core points:
  • Correlation does not prove causation
  • Correlation is consistent with multiple candidate causal inferences
  • To prefer one causal claim over another requires: a) the identification of alternative causal claims; b) making reasoned arguments about why the alternatives are less likely to explain the observed association
  • Causal effects on big dependent variables are more likely to result from other big variables; and if a dependent variable is small, causal effects are likely to flow from other small variables measured in the same setting or context.
This all got me thinking: Are there any interventions which could exert a causal effect to improve the reasoning about causality in popular newspapers?


  1. This is one study that has really taken off in the public audience. Your exercise (e.g., inquiry, curiousity, knowledgeable questioning= science!)is one that we all must do. Another issue that needs to be addressed --what is 5 points, and 2.8 points for that matter, mean on an IQ test? These tests--likely, cognitive tests, typically have a mean of 100, rendering 2.8 points quite meaningless.

  2. Hi Arkham,
    Thanks for your comment. I agree there is also an issue with what a 5 point or a 2.8 point difference in IQ actually means.
    The standard deviation on an intelligence test is typically 15. Thus, 2.8 / 15 would be a difference of approximately 0.2 standard deviations. In the language of psychology, that would typically be called a small effect. However, given the nature of IQ and how resistant it typically tends to be to environmental manipulation, I would find a 2.8 point effect quite substantial. The only thing is that I'm not convinced that rival alternative causal explanations have been ruled out.

  3. Hi Jeromy,

    The "Behind the Headlines" blog put out by the NHS has a thoughtful take on this study (and many others making the UK news).



  4. Maybe if the reporters got a spank before being able to submit something like that to their editor.

    Next week in the Herald Sun: "Spanking Leads to Higher Job Satisfaction - Male Staff Members on the Rise!" (*cough, cough*)

  5. Child buttock-battering vs. DISCIPLINE:

    Child buttock-battering for the purpose of gaining compliance is nothing more than an inherited bad habit.

    Its a good idea for people to take a look at what they are doing, and learn how to DISCIPLINE instead of hit.

    I think the reason why television shows like "Supernanny" and "Dr. Phil" are so popular is because that is precisely what many (not all) people are trying to do.

    There are several reasons why child bottom-slapping isn't a good idea. Here are some good, quick reads recommended by professionals:

    Plain Talk About Spanking by Jordan Riak,

    The Sexual Dangers of Spanking Children by Tom Johnson,

    NO VITAL ORGANS THERE, So They Say by Lesli Taylor M.D. and Adah Maurer Ph.D.

    Most compelling of all reasons to abandon this worst of all bad habits is the fact that buttock-battering can be unintentional sexual abuse for some children. There is an abundance of educational resources, testimony, documentation, etc available on the subject that can easily be found by doing a little research- visit

    Just a handful of those helping to raise awareness of why child bottom-slapping isn't a good idea:

    American Academy of Pediatrics,
    American Academy of Child and Adolescent Psychiatry,
    Center For Effective Discipline,
    Churches' Network For Non-Violence,
    Nobel Peace Prize recipient Archbishop Desmond Tutu,
    Parenting In Jesus' Footsteps,
    Global Initiative To End All Corporal Punishment of Children,
    United Nations Convention on the Rights of the Child.

    In 26 countries, child corporal punishment is prohibited by law (with more in process). In fact, the US was the only UN member that did not ratify the CRC.

  6. > Most compelling of all reasons to abandon this worst of all bad habits is the fact that buttock-battering can be unintentional sexual abuse for some children. <

    You're joking right? No, really, then what? Looking at a kid will become sexual abuse?

  7. There was a IQ test but what about a test on how they deal in public or how they study or maybe even how they do well in arts not all thing with IQ tests are right a person with a average IQ can be great at art or poetry or even building so have you check to see if people that were spanked have a better personality towards others or have clean police record then the one that were not. just a suggestion.