Full and true disclosure requires voluntary, complete and honest reporting of all material income and information. Full and true disclosure requires a voluntary, honest and complete revelation of all material facts and income by an assessee; 'true' denotes correctness and genuineness and 'full' denotes completeness. Mere production of books or other material that the assessing officer could discover does not amount to a true and full disclosure; the emphasis is on an earnest filing that declares all material and significant information relating to income.