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Under the economic benefit doctrine, a Taxpayer using the cash receipts and disbursements method of accounting must include in gross income currently any financial or economic benefit derived from the absolute right to receive property in the future that has been irrevocably set aside for the taxpayer in a trust or fund.
Generally, a cash basis taxpayer does not recognize income upon the receipt of property in the form of a promise to pay in the future. However, a cash basis taxpayer is taxed when the taxpayer receives an "economic benefit" from a right to receive property in the future. (Sproull v. Commissioner, 16 T.C. 244 (1951)) The economic benefit doctrine has also been applied to require inclusion in income of prize winnings when they are irrevocably placed in a fund to be paid to the winner at a later date. (Pulsifer v. Commissioner, 64 T.C. 245 (1975))
Under the constructive receipt doctrine, which is codified in § 451(a), income although not actually reduced to a taxpayer's possession is constructively received by him in the taxable year during which it is credited to his account, set apart for him, or otherwise made available so that he may draw upon it at any time, or so that he could have drawn upon it during the taxable year if notice of intention to withdraw had been given. However, income is not constructively received if the taxpayer's control of its receipt is subject to substantial limitations or restrictions.
The assignment of income doctrine provides that income is ordinarily taxed to the person who earns it, and that the incidence of income taxation may not be shifted by anticipatory assignments. However, the courts and the Service have long recognized that the assignment of income doctrine does not apply to every transfer of future income rights.

All these doctrines are applicable in determination of gross income under section 61.
Each doctrine has many court cases to clarify its application to different situations - and it would be incorrectly to say that any is easiest or hardest to apply.
The economic benefit doctrine is used when determining if the income received.
The constructive receipt doctrine helps to determine if income is considered as received (and therefore might be taxable) while it is not actually received or when the receipt is postponed by choice.
The assignment of income doctrine is used in determination of a person or an entity who received income and therefore might be subject of Tax Liability on that income.

Examples
The economic benefit doctrine - http://www.irs.gov/pub/irs-drop/rp-03-14.pdf
http://www.irs.gov/businesses/corporations/article/0,,id=134878,00.html
The constructive receipt doctrine - http://www.irs.gov/pub/irs-wd/01-0208.pdf
The assignment of income doctrine - http://www.irs.gov/pub/irs-drop/rr-02-22.pdf
http://www.zuberfamilylaw.com/News-Articles/Zuber_2000_Feb_Who-Pays-the-Tax.pdf

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