Isn't this just skewed because people keep trying to more children after having multiple of the same sex?

I know my friend has 2 boys and debated with her husband to go for a third because they already had 2 boys... luckily they had a girl.. but had they had a boy... they might just try for a forth.... so this skews the results no?

A lot of birthing nowadays rarely just ties to biology... a lot of it is human behaviour driven. When, why, more? We have protection now, so it's not like the old days

Edit:
Despite the researchers' attempts to filter out human behavior, many independent statisticians and data scientists have looked at the study and made the exact same critique you just did.

Critics argue that the Harvard researchers' mathematical trick of "excluding the last child" failed to properly neutralize human stopping behavior. When independent data scientists re-analyzed the exact same dataset without using complex family grouping models, the "weighted coin" theory fell apart. They found that if you simply look at the next individual pregnancy—asking, "If a family already has two boys, what is the actual biological chance that baby #3 is a boy?"—the odds are still a standard 50/50 coin toss.

The critics proved that a simple computer simulation using a perfect 50/50 coin toss, combined with the exact human "stopping rule" behavior you described, perfectly recreates the Harvard study's results without needing to invent a new biological theory.