A total of 512 children in Grades 1 through 6 received a conditional infere
nce task using causal conditionals (If cause P, then effect Q) and a genera
tion of alternatives task. The inference task used premises for which there
were fewer many possible alternative causes. Results show a steady age-rel
ated increase in uncertainty responses to the two uncertain logical forms,
affirmation of consequent (AC) and denial of antecedent (DA), and an increa
se in production of disabling conditions for modus ponens. More uncertainty
responses were produced to AC and DA with premises with many possible alte
rnatives. individual differences in inference production were related to nu
mbers of alternatives produced in the generation task. Results support the
idea that bath developmental and individual differences in reasoning can be
at least partially explained by differential access to knowledge stored in
long-term memory.