In the book How Should We Live by Roman Krznaric, in the chapter on Family, he states that it's a 'myth' that families used to be larger and include several generations all living under the same roof and that the nuclear family as we understand it (mom, dad, and their offspring) has been the dominant structure for several hundred years.
He cites a bunch of census data from both the U.S. and Britain, so I guess I'm only asking about those countries. But this is literally the very first time I've even heard a sniff of a rumor that multi-generational households weren't the norm for my ancestors, and that the whole nuclear family thing was a very 20th century phenomenon. Is this guy a quack, or what?
I haven't read the book, but am familiar with these data.
If your ancestors were from the US and/or UK, and/or much of Northwestern Europe, starting in the late Middle Ages, yes, sort of. People from this region did not, as a rule, have multigenerational extended-family households. Nuclear families were the norm. First marriage took place at a later age, usually in the 20s, because the newlyweds were expected to set up their own household, which required a certain level of financial independence. As a result there were fewer childbearing years for the couple than families in areas where marriage trended younger. It's called the Western European marriage pattern.
Do not, of course, discount the high levels of premodern infant/child and maternal mortality rates as they impact family size; those factors truly have changed, as has the modern easy availability of reliable birth control. Also there will be fluctuations in family size based on factors such as war, famine and economic depressions, and pestilence--the four horsemen of the apocalypse yield not only death, but fewer babies. You see this in 20th century US demographic data--lower fertility during the Great Depression and WWII, followed by the anomalous baby boom.
I'm afraid I can't help with your question, but would you recommend the book itself?