(A) Except as provided in subparagraph (C), an individual shall be considered to be disabled for purposes of this subchapter if he is unable to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than twelve months.

(B) For purposes of subparagraph (A), an individual shall be determined to be under a disability only if his physical or mental impairment or impairments are of such severity that he is not only unable to do his previous work but cannot, considering his age, education, and work experience, engage in any other kind of substantial gainful work which exists in the national economy, regardless of whether such work exists in the immediate area in which he lives, or whether a specific job vacancy exists for him, or whether he would be hired if he applied for work. For purposes of the preceding sentence (with respect to any individual), “work which exists in the national economy” means work which exists in significant numbers either in the region where such individual lives or in several regions of the country.

(C) (i) An individual under the age of 18 shall be considered disabled for the purposes of this subchapter if that individual has a medically determinable physical or mental impairment, which results in marked and severe functional limitations, and which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months. (ii) Notwithstanding clause (i), no individual under the age of 18 who engages in substantial gainful activity (determined in accordance with regulations prescribed pursuant to subparagraph (E)) may be considered to be disabled.

(D) For purposes of this paragraph, a physical or mental impairment is an impairment that results from anatomical, physiological, or psychological abnormalities which are demonstrable by medically acceptable clinical and laboratory diagnostic techniques.

(E) The Commissioner of Social Security shall by regulations prescribe the criteria for determining when services performed or earnings derived from services demonstrate an individual’s ability to engage in substantial gainful activity. In determining whether an individual is able to engage in substantial gainful activity by reason of his earnings, where his disability is sufficiently severe to result in a functional limitation requiring assistance in order for him to work, there shall be excluded from such earnings an amount equal to the cost (to such individual) of any attendant care services, medical devices, equipment, prostheses, and similar items and services (not including routine drugs or routine medical services unless such drugs or services are necessary for the control of the disabling condition) which are necessary (as determined by the Commissioner of Social Security in regulations) for that purpose, whether or not such assistance is also needed to enable him to carry out his normal daily functions; except that the amounts to be excluded shall be subject to such reasonable limits as the Commissioner of Social Security may prescribe. Notwithstanding the provisions of subparagraph (B), an individual whose services or earnings meet such criteria shall be found not to be disabled. The Commissioner of Social Security shall make determinations under this subchapter with respect to substantial gainful activity, without regard to the legality of the activity.

(F) Notwithstanding the provisions of subparagraphs (A) through (E), an individual shall also be considered to be disabled for purposes of this subchapter if he is permanently and totally disabled as defined under a State plan approved under subchapter XIV or XVI as in effect for October 1972 and received aid under such plan (on the basis of disability) for December 1973 (and for at least one month prior to July 1973), so long as he is continuously disabled as so defined.

(G) In determining whether an individual’s physical or mental impairment or impairments are of a sufficient medical severity that such impairment or impairments could be the basis of eligibility under this section, the Commissioner of Social Security shall consider the combined effect of all of the individual’s impairments without regard to whether any such impairment, if considered separately, would be of such severity. If the Commissioner of Social Security does find a medically severe combination of impairments, the combined impact of the impairments shall be considered throughout the disability determination process.

(H) (i) In making determinations with respect to disability under this subchapter, the provisions of sections 421(h), 421(k), and 423(d)(5) of this title shall apply in the same manner as they apply to determinations of disability under subchapter II. (ii) (I) Not less frequently than once every 3 years, the Commissioner shall review in accordance with paragraph (4) the continued eligibility for benefits under this subchapter of each individual who has not attained 18 years of age and is eligible for such benefits by reason of an impairment (or combination of impairments) which is likely to improve (or, at the option of the Commissioner, which is unlikely to improve). (II) A representative payee of a recipient whose case is reviewed under this clause shall present, at the time of review, evidence demonstrating that the recipient is, and has been, receiving treatment, to the extent considered medically necessary and available, of the condition which was the basis for providing benefits under this subchapter. (III) If the representative payee refuses to comply without good cause with the requirements of subclause (II), the Commissioner of Social Security shall, if the Commissioner determines it is in the best interest of the individual, promptly suspend payment of benefits to the representative payee, and provide for payment of benefits to an alternative representative payee of the individual or, if the interest of the individual under this subchapter would be served thereby, to the individual. (IV) Subclause (II) shall not apply to the representative payee of any individual with respect to whom the Commissioner determines such application would be inappropriate or unnecessary. In making such determination, the Commissioner shall take into consideration the nature of the individual’s impairment (or combination of impairments). Section 1383(c) of this title shall not apply to a finding by the Commissioner that the requirements of subclause (II) should not apply to an individual’s representative payee. (iii) If an individual is eligible for benefits under this subchapter by reason of disability for the month preceding the month in which the individual attains the age of 18 years, the Commissioner shall redetermine such eligibility— (I) by applying the criteria used in determining initial eligibility for individuals who are age 18 or older; and (II) either during the 1-year period beginning on the individual’s 18th birthday or, in lieu of a continuing disability review, whenever the Commissioner determines that an individual’s case is subject to a redetermination under this clause. With respect to any redetermination under this clause, paragraph (4) shall not apply. (iv) (I) Except as provided in subclause (VI), not later than 12 months after the birth of an individual, the Commissioner shall review in accordance with paragraph (4) the continuing eligibility for benefits under this subchapter by reason of disability of such individual whose low birth weight is a contributing factor material to the Commissioner’s determination that the individual is disabled. (II) A review under subclause (I) shall be considered a substitute for a review otherwise required under any other provision of this subparagraph during that 12-month period. (III) A representative payee of a recipient whose case is reviewed under this clause shall present, at the time of review, evidence demonstrating that the recipient is, and has been, receiving treatment, to the extent considered medically necessary and available, of the condition which was the basis for providing benefits under this subchapter. (IV) If the representative payee refuses to comply without good cause with the requirements of subclause (III), the Commissioner of Social Security shall, if the Commissioner determines it is in the best interest of the individual, promptly suspend payment of benefits to the representative payee, and provide for payment of benefits to an alternative representative payee of the individual or, if the interest of the individual under this subchapter would be served thereby, to the individual. (V) Subclause (III) shall not apply to the representative payee of any individual with respect to whom the Commissioner determines such application would be inappropriate or unnecessary. In making such determination, the Commissioner shall take into consideration the nature of the individual’s impairment (or combination of impairments). Section 1383(c) of this title shall not apply to a finding by the Commissioner that the requirements of subclause (III) should not apply to an individual’s representative payee. (VI) Subclause (I) shall not apply in the case of an individual described in that subclause who, at the time of the individual’s initial disability determination, the Commissioner determines has an impairment that is not expected to improve within 12 months after the birth of that individual, and who the Commissioner schedules for a continuing disability review at a date that is after the individual attains 1 year of age.

(I) In making any determination under this subchapter with respect to the disability of an individual who has not attained the age of 18 years and to whom section 421(h) of this title does not apply, the Commissioner of Social Security shall make reasonable efforts to ensure that a qualified pediatrician or other individual who specializes in a field of medicine appropriate to the disability of the individual (as determined by the Commissioner of Social Security) evaluates the case of such individual.

(J) Notwithstanding subparagraph (A), an individual shall not be considered to be disabled for purposes of this subchapter if alcoholism or drug addiction would (but for this subparagraph) be a contributing factor material to the Commissioner’s determination that the individual is disabled.