Judging by the prevalence of sin and the early age at which children usually begin, we should say it was natural. David seemed to have that opinion (see Ps. 51 :$). It was not much better before the fall. Adam and Eve do not appear, according to the account in Genesis, to have made much resistance to temptation. The fact of its being natural accounts for a new nature being necessary, as Christ explained to Nicodemus (John 3:1-21).