such as diamond availability and soil fertility. The results suggest that historically, ruggedness is strongly correlated with decreased income levels across the globe and has negatively impacted state growth over time. They note that harsh terrain limited the flow of trade goods and decreased crop availability, while isolating communities from developing knowledge capital. However, the study also demonstrated that the terrain had positive effects on some African communities by protecting them from the slave trade. Communities that were located in areas with rugged features could successfully hide from slave traders and protect their homes from being destroyed. The study found that in these areas rugged topography produced long-term economic benefits and aided post-colonial state formation. Effects of climate on productivity The impact that climate and water navigability have on economic growth and GDP per capita was studied by notable scholars including Paul Krugman, Jared Diamond, and Jeffrey Sachs. By using variables to measure environmental determinism, such as climate, land composition, latitude, and the presence of infectious disease, they account for trends in worldwide economic development on local, regional and global scales. To do so, they measure economic growth with GDP per capita adjusted to purchasing power parity (PPP), while also taking into consideration population density and labor productivity. Economic historians have found that societies in the Northern Hemisphere experience higher standards of living, and that as latitude increases north or south from the equator, levels of real GDP per capita also increases. Climate is closely correlated with agricultural production since without ideal weather conditions, agriculture alone will not produce the surplus supply needed to build and maintain economies. Locations with hot tropical climates often suffer underdevelopment due to low fertility of soils, excessive plant transpiration, ecological conditions favoring infectious diseases, and unreliable water supply. These factors can cause tropical zones to suffer a 30% to 50% decrease in productivity relative to temperate climate zones. Tropical infectious diseases that thrive in hot and moist equatorial climates cause thousands of deaths each year. They are also an economic drain on society due to high medical costs, and the unwillingness of foreign capital to invest in a sickly state. Because infectious diseases like malaria often need a warm ecology for growth, states in the mid to high latitudes are naturally protected from the devastating effects of disease. Climatic determinism and colonization Climatic determinism, otherwise referred to as the equatorial paradox, is an aspect