The "peanut butter paradox" (coined, I believe, by Andrew Weil) arises when one tries to determine which variety of peanut butter is the healthiest. There are basically two types of peanut butter -- "Conventional" and "Natural." By conventional I mean well-known brands such as Jif(tm), Skippy(tm), and Peter Pan(tm). These brands almost always contain fully hydrogenated oils (clearly labelled as "hydrogenated xxxxx oil"), which are artificially refined and saturated, most commonly from cottonseed or soybean oil. Full hydrogenation converts all of the oil into artificially saturated fat, making it rather unhealthy for two reasons: saturated fats clog arteries (often measurably within hours) and raise LDL cholesterol. Artificially hydrogenated oils usually don't contain cholesterol, but they raise it in the bloodstream when metabolized.
There are of course low-fat brands, but they still contain the artificially processed oil.
A significant majority of soybeans in the US are grown from Monsanto(tm) patented genetically modified seed. Monsanto's newest manipulation allows soybeans and rapeseed (the real name for the euphemistic "canola oil") to withstand large amounts of Monsanto's favorite in-house herbicide, RoundUp(tm). It's designed to take a larger dose without wilting. Like any poison, in large enough doses RoundUp will kill any vegetation. See www.roundupready.com/soybeans/ for an optimistic perspective on this... Soybeans alone demand over two thirds of the pesticides used on any crops in the US. Cotton also requires large doses of chemicals in order to be grown profitably. These pesticides inevitably end up in the oils pressed from soybeans and cottonseeds.
Most peanuts are stored in silos where they aren't given adequate ventilation. Molds of the species Aspergillus flavus and A. parasiticus grow on the nuts and produce aflatoxins, which are toxic to the liver and highly carcinogenic. The estimated risk of cancer caused by regular consumption of aflatoxins is significantly higher than the risk caused by exposure to pesticide residues from any produce from the grocery store. Natural peanut butter contains nothing more than pureed peanuts, often with some added salt.
Mainly due to the fact that conventional brands contain large proportions of added fat, along with the fact that they sell peanut butter faster and thus keep peanuts in storage for shorter periods of time, so called "natural peanut butters" that are hydrogenated oil free contain, on average, ten times more aflatoxin than conventional brands.
Supposedly, the brand Arrowhead Mills sun dries its peanuts to prevent moisture accumulation that would otherwise cultivate fungi. Since they appear to be the only company who claims to do this, they have, in effect, a monopoly on the solution to the peanut butter paradox. And no, damn it...I don't work for them! I haven't even bought their brand yet myself; as of now I'll stick with the far more delicious almond and cashew butters. Pistachios, pecans, peanuts, walnuts, and other tree nuts are the most suceptible to Aspergillus infection. Aflatoxin is present in all peanut products, including peanut oil.