Against parachute skepticism

  • Find all the relevant studies.*
  • Sort through them and throw out anything that looks dubious.
  • Weigh up the ones that find ‘pro’ and ‘con’ evidence, and decide how strong the cases are.

Parachute use did not reduce death or major traumatic injury when jumping from aircraft. (Yeh, 2018)

*I use ‘study’ and ‘experiment’ interchangeably below. Meanings differ subtly between fields; I’ll use both terms for ‘empirical evidence’.

From (Smith, 2003), the great paper on parachute use.
The inimitable XKCD.
  • Child development research is not the same as popular ‘parenting advice’. In some cases, popular advice often tells people the opposite of what the research says.
  • For non-US readers: I am not trying to make a point about any religion(s). A majority of male newborns are circumcised in the US.


I need you to do something. It’ll take twenty seconds, and you need to do it now, or you won’t be convinced by this article.


The Force of Evolution

Can you remember your first crush? Can you remember how it colored everything? You felt that way because evolution exerts huge forces on our psychology. We have strong drives to behave in ways that make us more likely to survive and to have children.

Developmental Plasticity

An individual organism can ‘grow up’ to have different adult forms depending on what environment it grows up in. This is called developmental plasticity. For example:

From (Lafuente, 2019)
  • The fruit flies reflect a general biological rule that smaller forms are better suited to higher temperatures. (See Bergmann’s rule and its developmental-plasticity version, the Temperature Size Rule.)
  • For the butterfly Bicyclus anynana, lower temperatures happen in the dry season, when eyespots would interfere with camouflage (Lyytinen, 2003).

Life History Theory

Evolution allows more than one way to ‘do well’. Life history theory studies the evolutionary strategies that different species use. The most important kind of variation is between fast and slow strategies.

From here. The ideas here date back to (Promislow, 1990).

Fast and Slow Strategies in Individuals

Life history theory has also found fast-slow trade-offs for individuals in a species (Réale, 2010). So within one species:

  • Some individuals follow faster strategies. They start having children earlier, have more children and invest less in them. They also live less long.
  • Some individuals follow slower strategies. They start having children later, have fewer children and invest more in them. They also live longer.
Developmental Plasticity II: The Revenge!

Where are the Switches?

In vertebrates, the most important behaviour-affecting switches seem to be located in the hypothalamic-pituitary-adrenal axis, more often called the HPA axis. This is part of the stress response system; in humans it looks like this:

In response to stress, the adrenal gland produces the hormone cortisol. More on that in a second.


What I’ve said above applies to all vertebrates, but we’re particularly interested in humans. To sum up:

  • Danger in the environment causes stress, which is reflected in increased cortisol levels.
  • In early life, large or sustained cortisol rises (& other similar changes) ‘flip switches’ in the stress response system.
  • These flipped switches result in different lifelong levels of hormones.
  • Those changes in hormone levels push an individual towards increased risk-taking, risky sexual behaviour and other parts of a faster evolutionary strategy.

The Nature of Danger

I’ve outlined how humans react to dangerous early experiences by switching to fast life strategies. But: what counts as dangerous?

Neonatal Circumcision — the Experimental Evidence

Until recently, we had no experimental data on long-term psychological effects of circumcision. However: circumcision is known to cause enormous increases in the cortisol levels of newborns (Gunnar 1981).

From Gunnar (1981). Note broken y-axis.

Miani (2020)

The first substantial study to test these predictions was Miani (2020). I’ve bolded their key predictions about the long-term psychological effects of circumcision:

Note Miani (2020) look at sensation-seeking as a proxy for risk-taking attitudes; the two are closely related.
  • Bollinger (2011). Found a link to alexithymia, or difficulty idenfitying emotions.
  • Bauer (2013) and Frisch (2015). Found links to autism.
  • Ullmann (2017). Looked at only 9 circumcised men (!) and found no significant results.
  • Studies on sexual function, reviewed in (Morris, 2013) which concludes circumcision had no effect.


What did the paper find? This graphs sum things up:

Effect sizes are small. That doesn’t affect the argument in this article at all, but it may affect what you think about circumcision.
All the theoretical predictions were in the correct direction, although the results for trust and empathy were not statistically significant.

Parachute Skepticism

There is no stronger proof of a theory than this: it makes new predictions, and later experiments prove those predictions correct. For example, when Einstein formulated general relativity, he used it to make two new predictions; it’s was after those predictions proved correct that people believed the theory to be correct.

Partisans … systematically denigrate, depreciate and counterargue evidence that is contrary to their political views but accept uncritically the supportive evidence.

In the social sciences, you can reject any research in a seemingly scientific manner. Saying ‘causation is not correlation’, talking about the replication crisis, pointing out possible confounders and, of course, parachute skepticism will let you disregard any paper.

Which theories work?

This article has been about how you should not read science. I’d like to close by telling you more about how you should read it. Unfortunately, that’s just hard. I’ve written about the importance of theories, but not all theories in social science are backed by solid evidence. There’s a world of difference between

  • theories that only ‘explain’ pre-existing data
  • theories that have made new predictions, later proven correct

Afterword: Reading about child development…

I know some are readers are here because you’re interested in how children develop, so let me close by giving you some pointers to further reading.

…or not reading about child development!

In case you’re a parent, and that seems overwhelming, let me say this. You don’t need to read deeply into child development to be a good parent. Just as children have evolved to survive, you’ve evolved to help them survive. Babies will let you know when something seems threatening to them!* If you follow your instincts and soothe them, things are likely to turn out fine.

Only the Grinch could be skeptical of this parachute.



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Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.