An influential study found the opposite, but replications and meta-analysis show the effect is small and inconsistent.
What the evidence shows
Ophir, Nass & Wagner (2009) reported that people who habitually juggle many media streams were actually worse at filtering distraction and switching tasks — the opposite of the assumption that practice makes multitaskers nimble. The finding fed a wave of worry about attention in the smartphone era.
Subsequent replications and a meta-analysis (Wiradhany & Nieuwenstein, 2017) found the association between heavy media multitasking and poorer attention was small and not always reproducible, with signs of publication bias. So the reassuring claim (multitaskers are better) is not supported, but neither is the alarming one (multitasking wrecks attention) firmly established. The honest verdict is a weak, uncertain relationship that does not justify strong conclusions in either direction.
Sources
Ophir, E., Nass, C., & Wagner, A. D. (2009). Cognitive control in media multitaskers.
Proceedings of the National Academy of Sciences, 106(37), 15583–15587
Heavy media multitaskers performed worse on filtering distraction and task switching than light multitaskers.
DOI: 10.1073/pnas.0903620106 →Wiradhany, W., & Nieuwenstein, M. R. (2017). Cognitive control in media multitaskers: Two replication studies and a meta-analysis.
Attention, Perception, & Psychophysics, 79(8), 2620–2641
The link between media multitasking and worse distractor filtering was small and not consistently replicated.
DOI: 10.3758/s13414-017-1408-4 →