Car Seats as Contraception
Second-order effects are everywhere.
Researchers Jordan Nickerson and David Solomon, for example, found that US car safety seat laws have resulted in a lower birth rate:
Since 1977, state laws have required children to be restrained by car seats, with the age limits gradually rising over time. The problem for potential parents is that many cars can accommodate two, but not three, car seats in the back row. Car seat laws thus require three-child households to purchase larger cars, indirectly raising the costs of having a third child.
Their paper, Car Seats as Contraception, goes on to suggest that the laws haven't impacted families without a car or with only one child of car seat age.
The effects of this legislation are potentially significant: Nickerson and Solomon estimate 145,000 fewer babies have been born in the U.S. since 1980 as a result.
There are a lot of variables at play in trying to draw a clear cause/effect relationship here. But the broader implication from a study like this — that nothing happens in isolation — is certainly true.
Everything we do is part of a larger system, and it's impossible to change one thing without incurring secondary consequences.
The challenge is thinking far enough ahead to anticipate the full effects of our actions.
See also: Nuclear-Powered Pacemakers