What are the two types of checks on population described by Malthus?

Humanity has only a limited ability to voluntarily reduce the growth of its population (through ‘preventive checks’ such as postponing marriage or practising sexual abstinence or celibacy). Malthus believed, therefore that ‘positive checks’ to population growth – in the form of famines and diseases – was inevitable because they were nature’s way of dealing with the imbalance between food supply and increasing population.