Table 2.

Sequence of criteria for the inclusion of F₁ data and corresponding sample sizes.

criteria for inclusion	no.	use in analysis
all F1 generation	4515
not firstborn	3811
not a twin	3315
elder sibling not a twin	3100
survival to age 15 known	2769
lifespan of elder sibling known	1862
elder sibling sex known	1854
data available for potential confounders	1842
birth order <11	1765	core data: analysis (i)
survived to age 15	1081
tracked for reproductive lifespan	716
social class known	656	subset 1: analyses (ii), LRS and (iii), fecundity
produced at least one child	557	subset 2: analysis (iii), offspring survival rate