Drop In Fertility Research Articles

The economic explanation of fertility changes in the United States

Abstract It is widely assumed that fertility varies positively with economic conditions. Actually this assumption receives little support from the historical record. For a century before 1930 fertility declined while the economy expanded and real incomes rose. Then for nearly three decades fertility and incomes fell and rose together. Since 1960 they have again moved in opposite directions. Clearly, no simple generalization about their relation will hold water. More sophisticated explanations are based on relative rather than absolute incomes. Banks suggested that the downturn in English fertility in the 1870's might have occurred because standards of middle-class consumption rose faster than middle-class incomes, but he found the evidence inconclusive. To reconcile the post-war baby boom in the United States with earlier experience, Easterlin has argued that fertility is determined by the relationship between the income of couples in their twenties and the income of their parents ten to fifteen years earlier. Among the weaknesses of this theory as applied to U.S. experience are its failure to explain the sharp drop in fertility, including that of native white urban women, in the 1920's; the fact that fertility rose most in the baby boom at the higher socio-economic levels where incomes rose least; and the sharp decline of fertility after 1962 in spite of the favourable trend of incomes, including those of younger people. The broad conclusion is that while couples no doubt do consider income, employment opportunities, etc. in deciding how many children to have, such considerations have had a relatively minor influence on changes in fertility, which for the most part have been the result of changes in attitudes. Even the post-war baby boom was a result not only of higher incomes and full employment but also of a shift in attitudes toward family size, particularly among the better-educated, economically better-off sections of society.

On the momentum of population growth

Abstract If age-specific birth rates drop immediately to the level of bare replacement the ultimate stationary number of a population will be given by (9): multiplied by the present number, where b is the birth rate, r the rate of increase, the expectation of life, and R0 the Net Reproduction Rate, all before the drop in fertility, and μ the mean age of childbearing afterwards. This expression is derived in the first place for females on the stable assumption; extension to both sexes is provided, and comparison with real populations shows the numerical error to be small where fertility has not yet started to drop. The result (9) tells how the lower limit of the ultimate population depends on parameters of the existing population, and for values typical of underdeveloped countries works out to about 1. 6. If a delay of 15 years occurs before the drop of the birth rate to replacement the population will multiply by over 2. 5 before attaining stationarity. The ultimate population actually reached will be higher insofar as death rates continue to improve. If stability cannot be assumed the ultimate stationary population is provided by the more general expression (7), which is still easier to calculate than a detailed projection.

Some social determinants of recent trends in British fertility.

SummaryThe contrast in birth rates in England and Wales between the periods on either side of the peak in 1964 is described and its components discussed. The drop after 1964 is mainly due to a drop in marital fertility at all ages, and especially the youngest. There is, however, as yet no evidence for a fall in the completed fertility of recent cohorts although their birth intervals are clearly lengthening. The mechanisms of such a change are described and their relative importance suggested. It is concluded that ultimate responsibility for the changing tempo of recent British fertility lies with fluctuations in the economic situation of the country.

Sperm competition and its evolutionary effect on copula duration in the fly Scatophaga stercoraria

The normal pattern of egg production in female dung flies consists of laying several successive egg batches of approximately equal numbers of eggs; although the number of eggs per batch is directly proportional to female size, the egg size remains constant. After a single insemination, a female can lay at least four batches without any drop in fertility. After a double mating in which the sperm from one male is labelled by exposure to 10 krad irradiation dose, the second male fertilizes 81·4 per cent of the eggs provided no further mating intervenes. After a double mating in which the second is interrupted, the competitiveness of the second mating can be shown to increase most steeply during the first half of copula. When females are mated immediately before laying (as in nature), matings labelled in various positions indicate that sperm from a given male can fertilize eggs in up to six successive batches, but that the percentage fertilization falls sharply as the number of batches (and hence number of subsequent matings) increases. With mated females, the last male to mate before oviposition fertilizes about 80 per cent of the succeeding batch irrespective of the number of previous matings. A model, which assumes that sperm from matings previous to the one before oviposition will compete for the remaining 20 per cent of the batch in the same proportionate relationship as they did for the previous batch, fits well with the experimental observations. Two patterns are therefore distinguishable for calculating the total egg gain of a male, depending on whether he mates with a virgin or non-virgin female. By assessing the chances of take-overs (where a second male ousts the male originally paired to a female) from field data, their effect is included in the model and the maximum, minimum, and probable average egg gains are calculated for males mating with females in different stages of their reproductive cycle. Egg gains with mated females could be increased by an increase in copula duration, but this would decrease the rate of capture of new females and also increase the probability of take-over. By using the model to assess the fertilization rate (eggs/min) achieved by males with varying copula durations in the present population and theoretical populations where the duration is both greater and smaller than at present, it can be predicted that selection would favour a value close to that observed. When the effect of virgin females is included, the fit is even better. Matings with post-oviposition females are much shorter than normal and the predicted optimum duration is very close to that observed.

Epidemiologic and Field Observations on Avian Influenza in Minnesota Turkeys

Avian influenza virus (AIV) was detected in Minnesota turkeys by virus isolation and by hemagglutination-inhibition (HI) tests with A/Turkey/Wisconsin/66 virus used as the HA antigen. The virus isolated appears closely related to the A/Turkey/Wisconsin/66 virus. The disease was seen primarily in turkey breeding flocks. The characteristic observed most commonly was a precipitous drop in egg production, sometimes to below 10% within 5 to 10 days. There was also a concurrent drop in fertility and hatchability. Many of the hens exhibited a depressed, listless attitude. Mild respiratory signs were noted in some outbreaks but not in others. All flocks remained serologically negative to HI tests for Newcastle disease and to complement-fixation tests for ornithosis. Several serum plate reactors to Mycoplasma gallisepticum (MG) were found early in the disease, but all flocks were considered negative on the basis of the HI test. All flocks were positive for Mycoplasma meleagridis by the serum plate agglutination test.

The Effect of Oxytetracycline on the Performance of Turkey Breeder Hens

LIMITED information is available on the effects of tetracycline antibiotics on the performance of turkey breeder hens. Several reports related to the use of antibiotics in the feeds of commercial laying hens and heavy chicken breeders have been published during the past few years (Carlson, 1959; Heywang, 1959; O’Neal and Savage, 1959; Ryan et al., 1961; Eoff et al., 1962). In general, these reports have shown improvements in egg production and feed efficiency due to the feeding of antibiotics.White-Stevens et al. (1955) reported that the continuous feeding of 100 to 200 grams of chlortetracycline per ton of feed to Beltsville Small White turkey breeders significantly increased eggs per hen housed and poults hatched per hen housed. With turkey breeder hens exhibiting a fertility problem, Green et al. (1963) observed in two experiments that tetracycline antibiotics administered through the feed or by injection prevented the precipitous drop in fertility observed …

The Emergence of Differential Fertility in Urban Egypt

Egypt is a classic example of a modernizing society that has already experienced a marked drop in fertility. Projections for the future need to now consider the time when Egypt will enter the next phase of demographic transition that is a declining birthrate. If demographic theory is correct - the beginning of an overall decline in fertility will be signaled by the emrgence within the urban population of fertility differences by socioeconomic class - the presence or absence of fertility differentials in urban Egypt needs to be examined if an estimate is to be made of how close this country is to ending her period of explosive growth. Data from the Census of 1960 concerning the number of her period of explosive growth. Data from the Census of 1960 concerning the number of children ever born by age of wife within given duration-of-marriage classes; education of wife within the same duration-of-marriage classes; and ocupation of husband by duration-of-marriage class are analyzed. The analysis reveals that education appears to be a most significant variable in the fertility of urban Egyptian women. There is a clear inverse relationship between the educational attainment of wives and the number of children born by them. Among women married for less than 5 years the strength of this variable is almost obscured by the operation of what appears to be a universal pattern of immediate unspaced childbearing regardless of educational level. However among women married for longer duration differences in education are increasingly reflected in fertility differentials. It appears probable from the data that at least among women married less than 20 years those who married between the ages of 20-25 had the highest fertility rates while those marrying later had significantly lower rates. Regarding occupation of hysband ranking by class is inversely correlated with fertility within each of the duration-of-marriage intervals although the range of variation is not nearly so great as that apparent when the independent variable was the wifes educational level.

A comparison of the production of F1 and F2 Border Leicester × Merino ewes

A comparison has been made of the production characteristics of 255 F1 and 288 F2 Border Leicester x Merino ewes. These sheep comprised a total drop of ewes born in a breed establishment trial in March-April 1960. There were no significant differences between the means of each group of ewes for the following characters : 18 month body weight, mutton score, fleece and breech score, yield, staple length, crimps per inch, fibre diameter, follicle density, ratio of primary to total follicles, and wool colour and character. The F1 ewes had significantly heavier greasy and clean fleece weights and showed significant though only slightly greater face cover scores. There was no significant increase in the variation of any character studied, in the F2 ewes. It is considered that these results are reasonable when the nature of inheritance of production characters, and their susceptibility to environment are considered. On the other hand the variance of horn growth, which is controlled by a few major genes, increased considerably in the F2 ewes. There was a large (40 per cent) and significant drop in fertility in the F2 ewes brought about by a drop in the number of wet ewes and multiple births, and an increase in lamb losses. The significance of these findings in relation to crossbreeding and breed establishment is discussed.

The Effect of Irradiation on Reproduction in Two Strains of Tribolium castaneum Herbst

The effects of x radiation on the reproductive performance of two strains of Tribolium castaneum were measured. The two strains differed in body weight, one strain being two and one-half times as heavy as the other. Females of the heavier strain were more resistant to somatic damage than were females of the light strain. Genetic damage, as measured by a reduction in fertility of irradiated individuals, was not so great in the heavy strain as in the light strain, especially when both parents were irradiated. When only one parent in a mating was irradiated, no sex differences in response to increasing dose were found. When both parents of a mating were treated, since the effective dose was doubled, there was a greater increase in genetic damage than if only one parent were treated. The effect of irradiating both parents can be predicted from the results obtained when only one parent is irradiated. The response curve for somatic damage showed a linear decline at high doses (1000 and 5000 r) but no significant effect at low doses (0 to 800 r). The response curve for genetic damage, as measured by an orthogonal polynomial analysis, was sigmoid in shape with a linearmore » decline at low doses and a curvilinear decline at the high doses. The genetic response curve over all doses can be fitted to a function of the type Y = a + bX + cX/sup 2/, where the different coefficients of X and X/sup 2/ change with the strain. The coefficients also depend on whether one or both parents are treated. The drop in fertility at doses over 5000 r was an average of 66.9% for irradiated foundation adults (summed over mating combinations) and an average of 58.8% for irradiated selected adults, (auth)« less