Kohlberg believed that we cannot attain the highest level of moral reasoning until we first reach Piagets stage of: postconventional operational thought. concrete operational thought. formal operational thought. sensorimotor thought.

Formal operational thought.

Explanation:

Kohlberg did some studies on morality and proposed a theory on moral development.

Kohlberg defined three different levels of morality (and each level consists in 2 stages).

These levels are

In the preconventional level, the person's morality is externally controlled and therefore acts based on what the punishment will be (1st stage) or based on how good will it be for him/her to act in some way (2nd stage)

The conventional level stages also have to do with external controls but now the person thinks in function of ensuring positive relationships and societal order. The third stage here depends on the approval of others. The fourth stage has to do with accepting rules because they preserve the society order  and functioning.

The postconventional level stages are defined in more abstract principles or values. The 5th stage sees the world as a place with different values that must be respected. Finally the 6th stage is based in universal ethical abstract principles.

Jean Piaget was a psychologist who developed a theory on cognitive development from birth to adolescence according to which people go through different stages in their process of thinking developing a more mature and rational thinking as they grow.

The last stage according to Piaget is the formal operational stage which starts at age 12 and involves an increase in logic and an understanding of abstract ideas and principles. Before this stage, kids cannot understand abstract concepts such as justice.

According to Kohlberg, a person couldn't reach the postconventional level of moral reasoning (which involves abstract principles)  if they weren't at the formal operational stage which is when they could be able to understand abstract principles.