"It is difficult to predict, especially the future" - (I think) Niels Bohr. The long-term future is hard to predict. If you are giving me money today, how much

"It is difficult to predict, especially the future" - (I think) Niels Bohr. The long-term future is hard to predict. If you are giving me money today, how much should I give you back 30 years from now? 4% annualized? 5% annualized? 1% annualized? or -1% annualized? All of these are valid numbers based on reasonable assumptions that might be encountered over those 30 years or so. IMHO, "defined benefits" where certain benefits are guaranteed by a public or a private plan decades into the future are a bad idea.