the "cold winters theory" of the evolution of human intelligence, which postulates that intelligence evolved to greater degrees as an evolutionary adaptation to colder environments. According to this theory, cold environments produce a selective pressure for higher intelligence because they present cognitive demands not found in warmer environments, such as the need to find ways of keeping warm, and the stockpiling of food for winter. James Flynn has criticized this theory as being inconsistent with the global distribution of IQ scores. If the theory were correct, the people of Singapore, who originated primarily from China's southern Guangdong province, would possess a lower average IQ than the people of mainland China, when in fact the reverse is true. In 2012 Scott A. McGreal, writing for Psychology Today, described it as a just-so story, saying the theory fails to account for challenges specific to warmer environments, and also does not explain why hominids who evolved for millions of years in colder environments (such as Neanderthals and Homo erectus) did not also evolve similar intelligence. In IQ and the Wealth of Nations (2002), Lynn and Vanhanen argued that differences in nations' per capita gross domestic product (GDP) are partially caused by IQ differences, meaning that certain nations are wealthier in part, because their citizens are more intelligent. K. Richardson wrote in the journal Heredity that "an association between IQ and national wealth is hardly surprising, though its causal direction is the opposite of that assumed by L&V. But I would not take the 'evidence' presented in this book to serve arguments either way." Other economists who reviewed the book also pointed to numerous flaws throughout the study, from unreliable IQ statistics for 81 of the 185 countries used in the analysis, to insecure estimates of the national IQ in the remaining 101 countries in the sample that did not have published IQ data. This was in addition to the highly unreliable GDP estimates for present-day developing countries and the even more unreliable historical data estimating GDP and national IQ dating back to the early 19th century, well before either concept even existed. Even the data on the 81 countries where direct evidence of IQ scores were actually available were highly problematic. For example, the data sets containing Surinamese, Ethiopian, and Mexican IQ scores were based on unrepresentative samples of children who had emigrated from their nation of birth to the Netherlands, Israel, and