Someone pointed out a few good ways to improve my methodology in calculating the likelihood of Jesus’s family (others, not so much but I’ll address those as well).
First, in Zorn’s study, the standard deviation was actually 2.5 necessitating a re-working of the model. Also if we assume that the Gospels were describing Jesus’s siblings symbolically as followers, then it meant he was thought to be an only child and the probability of that (a family of three or less) is what needs to be tested, not necessarily the probability of not being a family of 9+.
Taking all this into account, I came up with this:
The probability of having 6+ siblings is 3.6% while being a single child was slightly over 27%.
In other words, it’s about 8 times more likely that, given the conditions of the time, Jesus was portrayed as a single child with figurative siblings. As Zorn himself points out, houses were only designed for 2-7 people, but, in his estimation, more people lived there then was possible to hold.
Limit on Family Size
As mentioned previously, it’s also possible that no one out of the average family compounds could have had more than seven children, as Meyer’s estimates:
For the nuclear families…that were present in family compounds, a maximum of seven persons can be estimated, although that supposes a reproductive rate higher than what might in fact have existed, given the high number of infant mortalities…and the risk of maternal death at childbirth.
That means our lowest estimate is a zero chance of such a family being possible.
The More Things Change
Someone else challenged the notion that the ancient economy stayed the same on the basis of regional differences. This is a mistake: regional differences and the growth of the economy are two different things. While regions can vary because of their different capacities, their growth rates remained flat until the advent of industrialization.
For instance, when the economic historian Gregory Clarke said:
the average person in the world of 1800 was no better off than the average person of 100,000 BC.
He’s talking about it in terms of growth, not that the entire world was equal and that everyone on the planet had the same amount of children. This stagnant economy (and thus identical capacity to have children) would have applied to Israel as well.