Figure 4. Firstborn sons start reproducing earlier, probably due to their greater access to resources.

(a) Age at first reproduction as a function of intra-sex birth-order category: middleborn sons began reproduction later than firstborn sons (F_2,560 = 6.54, p = 0.002). No such effect was found among sisters (F_2,564 = 0.78, p = 0.5, not shown in the figure). (b) Proportion of men who owned a land among first-, middle- and lastborn sons: firstborn sons were twice more likely to become landowners (χ² ₂ = 25.71, p<0.0001).