Motivation is certainly a factor in how anyone applies his nature. But motivation, itself, has several facets; some of which are clearly nature driven, as are some of the factors that will tend to externally motivate different individuals.
For example, you can expect a person with great physical skills to be more motivated--all else being equal--to develop those physical skills. You can expect a person with an analytic mind bent, to be more motivated towards intellecutal self-education, than one with other tendencies.
But there is also the factor of external motivation, and here the nature aspect takes on a somewhat different thrust. Certain peoples, for example, are much more internally motivated than others; but the value systems that may come to drive those internal motivations, may well be externally provided--as by parents, in early childhood. Other peoples are more externally motivated, and will respond far more readily to their social associates at any given time in their lives, than to a lifetime value system.
I am keeping this very general, but the points should be obvious. If someone wants to challenge this, however, I can give examples. This is far more than conjecture.
William Flax Return Of The Gods Web Site
If think motivation is picked up from one's family and peers. One may have abilities that allow an interest in certain areas.
But considering the importance of motivation to the development of one's potentials, if motivation is genetically determined, then the liberals are right, and where a person is is not his fault but formed beyond his choice and control.