werty - not sure if that's always the case - the UK and most other European countries had very large families pre 20th century often 8-12 was the norm, although this was partially due to the mortality rate and poverty was very high.
The norm now is around 2 children per household and obviously everyone is far better off - even the so called poor in the UK do extremely well by any other time in histories standards.
Not so in Third World countries with huge amounts of offspring with high mortality rates.