Otter's Rock

Humans are a bit more complicated that one pregnancy per year in order to deal with unpredictable circumstances, so I think your scarcity theory might be a good one.

Technically, a human woman can have about one baby per year but in reality, that rarely happens because breastfeeding works as a contraceptive unless you have enough good-quality food to both feed one infant and grow another, which is historically rare on a population-wide level. Beyond that, there's natural genetic variation like PCOS - women with PCOS find it easy to gain weight and difficult to conceive, but when they lose weight (current studies are saying 10% of their bodyweight or more, but this varies) the condition often recedes and conception is more likely. When does this happen? During famines, when most women are at a low bodyweight and unable to conceive. So even when most women can't conceive, a sub-group are having babies to carry on the species; while when conditions are good, that sub-group isn't conceiving. So maybe Vulcan went through something so catastrophic, or cycles that are so catastrophic, that children younger than seven and children without at least two intensely dedicated parents die, so over time the slow reproducers beat out the fast reproducers, and the closely-bonded pairs were more successful than the more widely procreating individuals.