It may be one of the most hardest, most important questions to answer in health policy right now: What will happen to premiums in 2014, when millions of Americans flood into the insurance markets?

It’s a question hugely important to the Affordable Care Act’s success. If Americans balk at the price of insurance, they may opt-out of the program, reducing the size of the insurance expansion.

Some argue that premiums will spike. Others think they won’t. The Obama administration, for its part, thinks there will be a bit of a mix; some will see their costs rise and others will not.

“Women are going to see some lower costs, some men are going to see some higher costs,” Health and Human Services Secretary Kathleen Sebelius told reporters Tuesday. “It’s sort of a one to one shift … some of the older customers may see a slight decline, and some of the younger ones are going to see a slight increase.”