Highlighting

Highlighting is one of the most popular learning techniques practiced by students (just look at a second-hand textbook and chances are it will be filled with highlighted or underlined text). It's easy to see why; it requires almost no effort (beyond reading the text itself) and it makes one feel like you are actively learning (e.g. you choose what to highlight). But is there any evidence to support its efficacy?

Unfortunately for students, not really. Cashen & Leitch (1970) found weak evidence that active highlighting aids memory in relation to answering exam questions about the highlighted material. This was replicated in another study by Leitch & Cashen (1972) but the effect was weaker still.

There have been several studies that found highlighting had no effect on subsequent test scores for children (Rickards & Denner, 1979), students with learning disabilities (Nist & Hogrebe, 1987) and typical students (Todd & Kessler, 1971).

Highlighting may even be detrimental to one's learning as students are prone to over-highlighting so the key information is lost in a sea of highlighting and is less distinctive (so therefore is less likely to be remembered, Lorch, Lorch & Klusewitz; 1995). It may also stop students from engaging in genuinely useful learning activities as they feel like they are effectively learning and so do not need to engage in other forms of testing.

The weight of evidence suggests that highlighting has a minimal (at best) effect on learning and in fact may have a detrimental effect overall.

References:
Cashen, V.M. & Leitch, K.L. (1970). Role of the isolation effect in a formal educational setting. Journal of Educational Neuroscience, 61 (6), 484-486.
Dunlosky, J.; Rawson, K.A.; Marsh, E.J.; Nathan, M.J. & Willingham, D.T. (2013). Improving Students' Learning with Effective Learning Techniques: Promising Directions from Cognitive and Educational Psychology. Psychological Science in the Public Interest, 14 (1), 4-58.
Leitch, K.L. & Cashen, V.M. (1972). Type of Highlighted Material and Examination Performance. Journal of Educational Neuroscience, 65 (7), 315-316.
Lorch, R.F.; Lorch, E.P.L. & Klusewitz, M.A. (1995). Effects of Typographical Cues on Reading and Recall of Text. Contemporary Educational Psychology, 20 (1), 51-64.
Nist, S.L. & Hogrebe, M.C. (1987). The Role of Underlining and Annotating in Remembering Textual Information. Reading Research and Instruction, 27 (1), 12-25.
 Rickards, J.P. & Denner, P.R. (1979). Depressive effects of underlining and adjunct questions on children's recall of text. Instructional Science, 8 (1), 81-90.
Todd, W.B. & Clemm, C.C. (1971). Influences of response mode, sex, reading ability, and level of difficulty on four measures of recall of meaningful written material. Journal of Educational Psychology, 62 (3), 229-234.