Indeed this is a prediction of Solow's exogenous economic growth models: there are diminishing returns for investment in either capital or labor alone, but line

Indeed this is a prediction of Solow's exogenous economic growth models: there are diminishing returns for investment in either capital or labor alone, but linear returns for investment in both. With a country which has had very low capital investment (later growth models separated capital and technology), you will see very high growth rates, which will experience diminishing marginal returns as additional investment is made. Ultimately, you will return to the point where investment in capital, labor or technology have equal returns to each other, so all must be invested in. It is argued that this is where developed nations are today.