Many subsequent researchers have further developed the CPH, most notably Elissa Newport and Rachel Mayberry. Studies conducted by these researchers demonstrated that profoundly deaf individuals who are not exposed to a sign language as children never achieve full proficiency, even after 30 years of daily use. While the effect is most profound for individuals who receive no sign language input until after the age of 12, even those deaf people who began learning a sign language at age 5 were significantly less fluent than native deaf signers (whose exposure to a sign language began at birth). Early language exposure also affects the ability to learn a second language later in life: profoundly deaf individuals with early language exposure achieve comparable levels of proficiency in a second language to hearing individuals with early language exposure. In contrast, deaf individuals without early language exposure perform far worse.