With no inclusion of numbers effect as a separate objective (N = 0 or αN = 0),
a two-objective SWF violates an equity-related axiom: a marginal increase in the
share of residents below the threshold brings about a decline in social welfare only
if the inequality between distribution weights is reversed, that is: ∂W/∂F(s) < 0 ↔
vb > vw. If each individual receives the same weight in society (after controlling for
vw and vb)4, the numbers effect can be re-expressed as αNN = α(N/P) ≤ αF(s)
(with P population size). In this case, the inclusion of a separate additive term
accounting for the uncompensated losers avoids the above violation as long as the
parameter α complies with the restriction: