To answer JVW's question: • On March 19, 1970 the First Presidency issued a statement on tithing: “members of the Church should pay one-tenth of all their interest annually, which is understood to mean income. No one is justified in making any other statement than this. We feel that every member of the Church should be entitled to make his own decision as to what he thinks he owes the Lord, and to make payment accordingly.”
• General Handbook of Instructions 34.3.1: “Tithing is the donation of one-tenth of one’s income to God’s Church (see Doctrine and Covenants 119:3–4; interest is understood to mean income). All members who have income should pay tithing.”
• “Increase” is the growth of asset value, offset by expenditures. Assets may increase or decrease in value depending on events unrelated to one’s efforts, and there may be no “increase” at all if one consumes all that they earn. Calculating tithing on "increase" is an Old Testament concept predating current economic structures and pay structures, before the world had monetary systems, banks, interest and inflation.
• “Income” is earned by one’s own labor or by investments.
I believe that the First Presidency’s use of the word “Income” is intentionally clarified and differentiated from “interest” and “increase”. To have chosen another definition would have incurred risks of lower tithing receipts.
It seems proper to follow the direction of current church leadership and tithe on “income”, not the Old Testament way of tithing on “increase”, or the 1838 interpretation of tithing the “interest” earned on one’s net worth. Clearly tithing is due on money received for your work, and what your investments earn. "...
Inheritances and gifts are clearly "increases" but they aren't "income" derived from "capital or labor", so I don't know what to make of that.