For each month, an estimate of inflation-adjusted monthly spend for a child born in every preceding month was calculated.

It was assumed that the monthly spend for each month was invested instead in Apple stock at the month-end closing price (adjusted for splits and dividends).

A $20 flat (non-inflation adjusted) trading fee was applied to each monthly purchase of shares.

If college costs were added, the same methodology applied by assuming monthly spend was equal to annual amount specified divided by 12. Note that for children not currently at the age of 18, college spend would not apply.

Total shares were multiplied by the last closing price of Apple stock to estimate total opportunity cost.

Cost of Raising a Child

Estimated spend per child was sourced from the Consumer Expenditure Survey by the U.S. Department of Labor conducted in 2005-06. The survey provides an estimate for the annual cost of raising a child in a dual-parent family up to the age of 18 for three differnet income brackets. This data is summarised on Wikipedia.

Estimating Cost per Month of Childhood

Estimates of annual child expenditure were divided by 12 to estimate a monthly amount. This amount was then adjusted by U.S. Department of Labor Bureau Statics published CPI (All Urban Consumers) to estimate the costs incurred in specific months. A table of this data can be found here.

Estimating Shares Purchased

Adjusted closing share price for Apple was sourced from Yahoo Finance. Prices have been adjusted for stock splits and dividends.