The psychology text Influence: Science and Practice describes the act of reciprocity as a trait in which a person feels obliged to return favors.
[2] Scientific malpractice involving shoddy research or data manipulation does occur in rare instances.
[4] The tobacco industry, for example, would publish their own internal research that invariably found minimal adverse health effects of passive smoking.
[7] Selection bias may result in a non-representative population of test subjects in spite of best efforts to obtain a representative sample.
Even a double-blind study may be subject to biased selection of dependent variables, population (via inclusion and exclusion criteria), sample size, statistical methods, or inappropriate comparators, any of which can bias the outcome of a study to favor a particular conclusion.