You don't have to be Bill Gates to strategically design wills, trusts, and one-shot gifts. A middle-class income is more than enough. Compared to the out-of-pocket cost of raising a child to adulthood, the baby bonuses in Quebec and Austria were small. They worked anyway. Since most people intrinsically desire children, modest incentives are often enough to convince them to have an extra baby.
You don't need to openly tell your children that their inheritance partly depends on how many grandchildren they give you. Don't hurt anyone's feelings. Just give generously to your children each and every time they enlarge your family. Then write a will that gives each of your children equal shares of whatever's left.
Another diplomatic way to reward fertility is to set up trusts for your grandchildren--and make your children the trustees. The parents will eventually use the trust to pay for expenses like college that otherwise would have come out of their own pockets. You're effectively giving more to the children who gave more to you. Yet officially, you're treating your children equally.
I'm following my own advice in my will. The vast majority of wills evenly divide the residuary estate between children. Mine evenly divides the residuary estate between (children and grandchildren). The draft text:
(2) If my spouse fails to survive me, I
direct that my residuary estate be divided into as many equal shares as may be
necessary in order to provide (i) one share for each of my children living or
dead and (ii) one share for each of my grandchildren surviving at my death and,
subject to the provisions of Article 2, I dispose of such shares as
(a) One of such shares shall be paid over
and distributed to each of my children and grandchildren surviving at my death;
(b) One of such shares shall be paid over
and distributed to the issue of each of my children who has predeceased me
leaving issue surviving at my death.
As usual, a trustee handles the funds for underage heirs.
Will my estate planning get me any additional grandchildren? I'll never know for sure, but I expect my eccentric incentives to bring at least one extra Caplan into existence.