Since 2002, use of the contraceptive pill has declined in favor of implanted contraceptive devices. In 2002, 19% of women aged 15-44 reported using the pill, while only 1.3% used IUDs. By 2017, pill usage had dropped to less than 14%, and 8.6% of women were using IUDs.
This, Pakaluk said, could be contributing to the sharp drop in unplanned pregnancies.
"These long-acting contraceptives tend to be much more immune to behavioral screw-ups. Even with the pill people are prone to contracepting badly and have a higher error rate leading to accidental but not necessarily unwelcome births, and these are disappearing."
"It's not a negligible percent, I don't think it is the whole story but I do think it could be enough to be dragging us down to the historic lows we are seeing."
Pakaluk said that while it is difficult to study, a shift in the way women approach pregnancy and contraceptives means that birth rates are increasingly subject to the expectations and experiences of generations raised in smaller families.
"One thing that should give us pause, and which I am really interested in examining more closely, is the effect of being around babies on adolescent wellbeing and mental health," Pakaluk said.
"If you live in a society in which the typical family has three or four children, the older children will be experiencing a young child into their teenage years. But if you move to an average of 1.5-2, no teenagers on average will live with babies – think what that means for their own likely fertility choices."
Experts have long warned about the wider societal and economic problems associated with declining birth rates, especially below the population replacement rate.
Last told CNA that the wider aspirations of society and politics to sustain and grow social welfare programmes depends on a demographic model opposite to current trends.
"The things we take for granted, let alone the things we aspire to do, in welfare, healthcare and so on, just do not work when you have an inversion of the population growth" Last told CNA.
Pakaluk agreed that there is widespread consensus on the economic and social problems associated with the long-term trend of lower fertility.
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"We see immediately that it is not socially optimal from any rational social planning perspective because you know you cannot support the generous social programs that we like to think are good for society," Pakaluk said. "Things like a decent social security system, MediCare, MedicAid, you just cannot sustain them in the long run with a total fertility rate of 1.7."
But if the wider problems associated with dropping fertility rates are well known, both Pakaluk and Last highlight widespread dissatisfaction at the personal level.
"While the wider societal problems are well known," Pakaluk said, "what is fascinating is that is seems that it isn't individually optimal either."
"What we do know, which is not often raised in media coverage, is that over the last several decades every survey in a Western country that asks women to describe their ideal family size – every single one everywhere – gives you a number about one child more than women end up having."
Last told CNA that these numbers need to be considered as a factor in the state of our society.
"What we are seeing is the constant 'fertility gap' between people's stated desire to have more than two kids and the reality that they tend to have less," Last said. "For a whole host of reasons, people aren't meeting their own expectations, and that has wider societal impact."