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پنجشنبه ، 13 مهر 1396 ، 13:33

Timing Effects on Fertility, Marriage, and Divorce

Robert SchoenRobert Schoen


In this chapter we examine fertility, marriage, and divorce, events that may never happen to an individual or that may happen more than once. The focus is on timing effects, in particular how the pace of cohort behavior impacts period measures, with feedback effects generally not considered. The chapter begins with a discussion of period and cohort perspectives on fertility. It then describes two approaches to adjusting period fertility for timing effects, one proposed by Bongaarts and Feeney (1998) and the other involving the Average Cohort Fertility. Those measures are compared in the context of population models and 20th century experience in the United States. Apparently paradoxical period cohort relationships in fertility are further examined using the dynamic fertility model underlying the Bongaarts-Feeney approach. Extensions of the Average Cohort Fertility approach to first marriage and to divorce are then described and discussed.

The Period-Cohort Contrast

In recent decades, a number of countries have had rates of fertility below the level needed for long term replacement, i.e. below a Net Reproduction Rate (NRR or R0) of 1. At the same time, the mean age at childbearing in many of those countries has been increasing. The combination of those two factors has renewed debate on the interpretation of period fertility measures and on the importance of changes in fertility timing. This chapter examines those issues in depth, drawing heavily on Schoen (2004).

The standard measure of fertility is the Total Fertility Rate (TFR). The TFR can be defined as the sum of the age-specific (female) fertility rates for all ages, and interpreted as the number of children the average woman would have under that specified schedule of fertility (in the absence of mortality and migration). If the birth rates of a particular year (or period) are summed, one gets a period TFR. The period TFR relates to a hypothetical (or synthetic) cohort, and does not describe the experience of any actual group of persons. A cohort TFR is found if the birth rates used follow the experience of an actual birth cohort.

A number of demographers, Norman Ryder (1969; 1980; 1986) foremost among them, have argued that the cohort perspective affords the best way to analyze fertility. Birth cohort measures describe the experience of real groups of women, exposed to the same historical events at the same point in their lives, who share a common reproductive past. Populations have fertility each year, but women have children, one at a time. Fertility theories generally seek to explain completed family size, i.e. cohort fertility. Over time, the cohort TFR (CTFR) has fluctuated less than the period TFR (PTFR), indicating that it is a more stable measure. To cohort advocates, a crucial limitation of the period TFR is that it does not distinguish between a change in the timing (or tempo) of cohort fertility and a change in the level (or quantum) of cohort fertility. For example, a decline in the PTFR of a particular year might mean that the cohorts active in that year are having fewer children, or simply that they have postponed childbearing without changing their ultimate completed family size.

The primacy of the cohort perspective has been undermined by challenges to both its conceptual basis and empirical support. Ni Bhrolchain (1992) saw “period as paramount”, noting that the cohort perspective implies a fixed-target view of decision making that is not consistent with observed behavior. Statistical analyses of fertility behavior have repeatedly shown that periods, not cohorts, account for most of the variation in fertility behavior. Characteristic age patterns have been observed in period fertility but, unlike the case in mortality, no such patterns have been found in cohort fertility. The typical pattern of change has been a rise (or a fall) in period fertility at all ages.

Those facts clearly demonstrate the importance of the period perspective, but they can be interpreted in ways much less critical of the cohort view. To say that cohort changes account for less variability in fertility just puts a different slant on the argument that the cohort TFR is a more stable measure. It is the meaningfulness of the measure, not its variability, that counts. What matters theoretically, and in many cases substantively, is completed family size. Women can achieve their long term childbearing goals in many ways, and one should expect that period circumstances will influence fertility timing. The process can be seen as analogous to a drive from one location to another. The driver proceeds toward a given destination, but at different speeds under different traffic conditions.

The argument that the cohort view embodies a fixed reproductive target is a serious criticism, because there is considerable evidence that there is no such target (cf. Lee, 1980). Yet the fact that women change their reproductive goals over time is not problematic unless an extreme cohort position is taken. Returning to the driving analogy, the initial destination can remain meaningful even if traffic conditions cause the driver to stop earlier or go further than first planned. Moreover, the final destination is often the fact of greatest importance. In sum, both the cohort and period perspectives are valuable in the study of fertility. To ignore either one can be unwise (see Section 6.7. below). Furthermore, given the present focus on timing, we need to compare and contrast period and cohort fertility behavior. The period perspective by itself is not enough because, as Bongaarts and Feeney (1998: 178) recognized, “a notion of ‘deferring’ or ‘advancing’ births necessarily refers at some level to cohorts.”

Source: Schoen, R. (2006) Dynamic Population Models, The Springer Series on "Demographic Methods and Population Analysis", The Netherlands: Springer, PP. 101-103.

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روش‌های تحلیل جمعیّت‌شناختی: این کتاب توسّط سه تن از جمعیّت‌شناسان نامی علم جمعیّت‌شناسی یعنی فرحت یوسف، جو. ام. مارتین و دیوید ا. سوانسون در چهارده فصل به رشته‌ی تحریر درآمده و در سال 2014 توسّط انتشارات اسپرینگر چاپ و منتشر شده است. دکتر حاتم حسینی و میلاد بگی کتاب را به زبان فارسی برگرداندند. ترجمه‌ی فارسی کتاب در 460 صفحه و شمارگان 1000 نسخه توسّط مرکز نشر دانشگاه بوعلی سینا در تابستان 1396 چاپ و منتشر شد. مطالب این کتاب به شیوه‌­ای سازمان یافته است که اجازه می‌دهد تا خوانندگان از یک سطح مقدّماتی به روش‎های پیشرفته‎تر تحلیل­‌های جمعیّت‎شناختی حرکت کنند. این رویکرد با در نظرگرفتن این نکته است که ممکن است کاربران ...

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