The cohort effect represents the similarity among a group of poeple due to the same events experienced during their developments. This effect is quiet important, because the environments of developments between generations of people differ a lot, which can dramatically interfere the evaluation of the development outcomes in research.
In my homeland, China, with the intensive economic development and social structure changes, this cohort effect is quiet influential. For example, my father and my uncle who were born during the famine in China from 1958 to 1961 shows deficiencies on physical development, especially on height. But as the generation raised during the development of economics, my cousin and I are both more then fifteen centimetres higher than them. And the same effect doesn't show on my mother and her generation, who were born after the great famine.
Furthermore, the social contexts of different generations differ a lot. In China, people like to sort themselves to different groups as 70s, 80s and 90s(born in 1970s, 1980s, 1990s). The 70s are marked with conformity, diligence and good luck of encountering the start of the great development of society. While 80s are also diligent. But with the inflation and more fierce competition, they are recognized as the generation facing the unpresedented social pressure. Then the 90s, born during the most developed time in China, are more individualistic and confident.
With the great impact from the cohort effect, researchers need to carefully eliminate the possible confounding variables to draw conclusions about the human development.