What is post modernism and what are the causes and effects of post modernism?

Postmodernism is a framework which seeks to reject or re-assess the knowledge and ways of knowing developed during the modern period. The scholars of the modern period emphasized the quantifiable nature of the world and worked with theory attesting to the concrete, quantifiable, and knowable nature of all things. In more recent years, postmodernism has arisen in the arts and social sciences out of the disillusionment with such hard theoretical thinking. Postmodernism emphasizes the subjective nature of knowledge and reality, and deals with the question of how we come to know what we know to be true.

The cause of postmodernism is quite simple-- the aforementioned disagreement with a purely quantifiable and predictable theory of how the world works. Such disillusionment may occur on a small or large scale, equally prompting one to re-address what has previously been taken for granted as truth. 

The effects of postmodernism are many. Knowledge and the means of attaining knowledge change throughout time, and postmodernism occurs one to take all knowledge with a grain of salt. How do we know what we know? Allow me to refer to an example I have previously outlined in answering on postmodernism.

If a  modernist weighs a sack of potatoes using standardized weights and determines it to be about one kilo, they  might argue that all sacks of potatoes of a similar size would also weigh about one kilo. The postmodernist, on the other hand, recognizes that the system of weights and measures used to determine the weight of the sack of potatoes has been previously established as an arbitrary means of quantifying weight. Weighing the sack of potatoes in the metric system has little value to the person who works with imperial weights and does not know conversion rates. Similarly, one kilo of potatoes might feel quite light to an adult, and quite heavy to a child. What's more, determining the weight of the sack of potatoes doesn't tell us much about its quality-- are the potatoes ripe or rotten? Is it many small potatoes, or a few large ones?

While this example is a little bit silly, I hope it helps you get a better understanding of the importance of subjectivity and personal experience in postmodernism. This emphasis on skepticism on previously established systems of knowledge is especially favored by social scientists seeking to re-address the often skewed opinions of their predecessors in their fields of study. There has been a significant shift in psychology, sociology, and anthropology from the observed, outsider perspective to the lived experience of insiders. For artists, postmodernism also prompts questions about the nature of art. Is art only something we can look at? Must it be beautiful? 

In many ways, postmodernism is about subverting the truths and means of attaining truth established during the modern period.