SLA research into the effect of instruction compares untutored learners in naturalistic settings with instructed or classroom learners. This research suggests that while acquisition processes in the two contexts may differ, learners all follow the same route of acquisition, that is, pass through the same developmental stages. These stages cannot be skipped, and instruction is only effective when pitched at a stage close to that of the learners. Such instruction does, however, increase the rate of acquisition and the level of ultimate attainment: classroom learners go farther faster.
Explicit instruction includes all types in which rules are explained to learners, or when learners are directed to find rules by attending to forms. Implicit instruction makes no reference to rules or forms (Doughty, 2003).
In a review of second language research on implicit learning, when learners are invited to infer rules from input, DeKeyser concludes that learners receiving explicit, deductive instruction tend to outperform controls in implicit, inductive learning conditions.
This finding supports the Fundamental Difference Hypothesis (Bley-Vroman, 1988), which explains child-adult differences in language acquisition by suggesting that:
Children use Universal Grammar and domain-specific learning procedures, while adults draw on native language knowledge and general problem solving systems.
DeKeyser is quick to point out that educators should conclude not that instruction should start early, but rather:
The instructional approach should be different depending on age: full-scale immersion is necessary for children to capitalize on their implicit learning skills, and formal rule teaching is necessary for adolescents and adults to draw on their explicit learning skills.
Read more: Principles of instructed second language acquisition (2008) by leading researcher Rod Ellis.