Description of the model variables
| Variables | Description |
| R | Proportion of male births; sex ratio at birth. |
| λ | Average number of children per family within a population. |
| Φ | Proportion of intervening couples, that is, couples reverting to sex selection. |
| Ψ | Proportion of intervening parents (excluding childless couples). |
| n | Birth order threshold of intervention: universal threshold in model A; minimum universal threshold in model B. |