What valid points did Thomas Malthus and Karl Marx make in their theories about population?

Malthus specifically stated that the human population increases geometrically, while food production increases arithmetically. Under this paradigm, humans would eventually be unable to produce enough food to sustain themselves. This theory was criticized by economists and ultimately disproved.

Essentially, Malthus was wrong on both counts: population growth and technical change. He did not specify the exact rate of population growth, but suggested that with abundant natural resources (as in The New World), population would tend to double every 25 years.Malthus specifically stated that the human population increases geometrically, while food production increases arithmetically. Under this paradigm, humans would eventually be unable to produce enough food to sustain themselves. This theory was criticized by economists and ultimately disproved.

a countries leader should be motivated by the common good of the country.