in fertility rate. The most notable examples of this phenomenon are accredited to the First and Second World Wars. Modeled by these examples, total war subjects individuals to intense social upheavals and a heavy psychological impact that forcefully prioritizes survival and economic stability over the need to reproduce for the duration of the conflict. Events like these subsequently pave way for an active effort to repopulate, such as the "baby boom" after the Second World War. Tempo effect Total fertility rate (TFR) is affected by a phenomenon called the tempo effect, which describes "distortions due to changes in the timing of births." John Bongaarts and Griffith Feeney have suggested that this tempo effect is driving the decline of measured fertility rate in the developed world. Specifically, the trend in developed countries of having children at later ages can cause the TFR to be underestimated. For example, as measured by the Human Fertility Database the United States adjusted total fertility rates have been at replacement levels since 1992. Type of partnership A study of the United States and multiple countries in Europe came to the result that women who continue to cohabit rather than get married after birth have significantly lower probability of having a second child than married women in all countries except those in Eastern Europe. Another study, on the contrary, came to the result that cohabiting couples in France have equal fertility as married ones. A large survey in the United States came to the result that married women had an average of 1.9 children, compared to 1.3 among those cohabiting. The corresponding numbers for men were 1.7 and 1.1, respectively. The difference of 0.6 children for both sexes was expected to decrease to between 0.2 and 0.3 over the lifetime when correcting for the confounder that married people have their children earlier in life. In the United States, those who cohabit without marrying had increased fertility when the male earns considerably more than the female. Frequency of sex Another explanation for falling fertility could be a reduction in the frequency of sex. For example, according to the Japan Family Planning Association 2016 survey, 47.3% of the men and 47.1% of the women had not had sex with their spouse in the previous month. High investment per child People are more likely in modern society to invest strongly in the needs of their children, such as offering them