Martin Cooper designed the first portable cellular phone in 1973, bringing mobile communication out of cars and into people's hands. The introduction of the "DynaTAC" phone in 1983 started popularizing mobile phones, leading to over 1 million US subscribers by 1990. Modern smartphones allow extensive multitasking through apps and mobile internet, stimulating people's desire to accomplish more on the go. However, some researchers warn that constant multitasking may negatively impact concentration.