The passion for marriage is so strong (thought Malthus) that there is no fear for the race; it cannot be over-controlled.
The gratification of the sexual instinct, and procreation were the same thing in the mind of Malthus.
But this is not so.
A physiological law makes it possible, in a large proportion of strictly normal women, for union to take place without fertilisation.

If it were possible to maintain an intermittent restraint in strict conformity with this law, it would control considerably the population of the world.
It is easier to practice intermittent than to practice constant restraint.
It is just here that Malthus failed to anticipate the future.

Malthus believed that "moral restraint" would lessen the marriage rate, but would have no direct effect on the fecundity of marriage.
A man would not put upon himself the self-denial and restraint, which abstinence from marriage implied, for a longer period than he could help.
The greater the national prosperity, therefore, the higher the birth-rate.

But prosperity keeps well in advance of the birth-rate; in other words, population, though it still _tends_ to, does not actually _press_ upon the food supply.
If the moral restraint of Malthus be extended so as to include intermittent moral restraint within the marriage bond, then, under one or other, or all of his three checks, vice, misery, and moral restraint, will be found the explanation of the remarkable demographic phenomena of recent years.
_Misery_ will cover deaths from starvation and poverty, the limitation of births from abortion due to hardship, from deaths due to improper food, clothing, and housing; and emigration to avoid hardship.
_Vice_ will cover criminal abortions, limitation of births from venereal disease, deaths from intemperance, etc., and artificial checks to conception.

Malthus included artificial checks of this kind under vice (7 ed.