In liberal academic circles, the "pro-active" innate behaviour of workers is "given", self-evident and determinant in society, as if Capitalism hadn't already done away with the workers' personal contribution by means of the labour market, wherein the workers' labour-power is bought by the Capitalist as a commodity. Such an innate behaviour appears to these folks as "determinant", as if the 'economic factors' did not shape the societal features — from commodity fetishism, consumerism and postmodern idiocy to egotism and humanistic indifference.
"To each according to his ability!", all the inherited politico-economic factors (ownership and established social roles) to be accepted and not questioned: this is the motto of modern neo-liberal political economy.
People, by nature, certainly have heterogeneous temperaments that lead them to have heterogenous 'choices', but — unlike what the Liberal law (and some vulgar post-Marxist) may think — such 'choices' are not completely 'genuine'. "Free" will does not exist. Will are strictly correlated to the dominant economic base and productive forces as well as the dominant superstructure (as modelled by the mode of production) of one society. People certainly 'choose' with their own 'subjectivity', but this subjectivity is strongly influenced by the way people make a living and relate to each other through their material surroundings.
People choose between a pre-arranged set of choices, whose quantity and quality is determined by the abundance/scarcity of one mode of production and the past unproductive/productive inheritance, in material and 'psychological terms' 
Whilst trying to define at which extent the natural temperament is 'flattened', repressed and curbed by the material scarcity of a mode of production, Marxism does recognise the 'biological' differences amongst the individuals. Exactly because it does, Marxism seeks to arrange the systemic resources so as to 'abolish' such differences in social and economic terms within Communism, therefore in the long run, once resource abundance is achieved through Socialist production. Communism, unlike meritocratic Socialism, focuses on the realisation of needs, on the human happiness.
Everybody may be endowed with different abilities, yet everybody shares the same categories of needs, which need to be satisfied for individual pleasure and happiness. Communism is the ultimate produce of human society; a produce for itself and its people.
The Capitalist is interested in the exchange value. He does not give a fig about the 'social need'; he cares only about the surplus value which he can appropriate in the form of profit-maximisation from the realisation of his sales on the market.