Based on a novel formal demographic method called born once—die once approach, our team is systematically investigating whether eight specifically known regularities of death patterns also can be found in birth patterns. Specifically, the born once die once approach allows calculating the “hazard of birth”, in analogy to the “hazard of death”. Thereby, we can capture and compare the full birth and death patterns with the same measure, which is an unexplored area of research.
First, we investigated whether birth patterns, just as death patterns, rise exponentially over adult ages. As our key first finding, we could verify that not just death, but also birth follows an exponential hazard. We consider this a substantial advancement because based on current knowledge, it was unexpected that birth can be modeled with the same mathematical model as death, which turns out to be the simple and mathematically convenient exponential function. Interestingly also from the perspective of aging research, we find that for both birth and death, the exponential pattern is observed over the age-range that is associated with respectively reproductive and actuarial senescence.
We consider this a major theoretical leap forward in the field of fertility analyses, not just because it increases the analytical power of modeling fertility by offering an analytically simple and universal parametric model. With the same, shared and simple model of the hazard of birth and death, our finding now enables developing novel formal demographic models and theory of fertility by importing and mirroring existing models and theory of mortality patterns.
Second, we investigated the trend in the mean age at birth in the front runner countries who experience, on average, the latest age at childbirth. Strikingly, we discovered a linear trend in the maximum mean age at birth that has been persisting over four decades. We argue that, together, the linear trends in maximum mean age at birth and at death reveal a shared constraint of aging. This constraint shapes and limits the potential postponement of birth and death to higher and higher ages.
Surprisingly, we also found that the slope of the linear trend in maximum mean age at birth falls right within the same ballpark as the rate of reproductive aging. This rate is reflected in the slope of the birth hazard function, discovered in our first project. More surprisingly, this value is also similar to the slope of the death hazard function, identified by previous research, and commonly referred to as the rate of actuarial aging. We are now digging deeper into this unexpected finding that birth and death seem to share the same pace of aging using a shared model of birth and death, that is, a model that applies to both processes equally and that can separate the senescent (aging) component of birth and death from its behavioral and environmental components.
Thirdly, we also investigated the postponement of birth by percentile measures, that is by interquartile ranges. Just as mortality researchers have discovered an "advancing front of old-age human survival", here we find that also birth patterns exhibit an advancing frontier of age-specific fertility contributions, visible in constant distances between birth survival percentiles.
In parallel to the empirical projects, we also developed an integrated analytical framework of fertility, which quantifies fertility patterns in a manner similar to mortality. It also translates between the novel and the traditional fertility frameworks. For the first time, this opens the door to study birth and death patterns together, as done in this project, and thereby connects two up to now almost disparate fields of demographic research.
With the new theoretical framework in place, we now can translate between different perspectives to measure fertility and can predict how strongly outcomes should diverge depending on alternative measures. This is currently helping us to double check the empirical results from our projects above to ensure that findings and conclusion we draw are not critically dependent on the specific measures applied or data that newly become available.
Taken together, our current findings support the main hypothesis underlying this ERC that birth and death are connect as expressed by mirrored regularities in population level patterns. These results encourage further pursuit of the principles of aging by defining, measuring and modeling aging as reflected in age-patterns of birth and death together.
