Higher magnitudes are represented approximately, and can be distinguished only with a large enough ratio difference; the higher the numerosities, the larger the ratio difference needs to be (Xu & Spelke 2000). For instance, chicks can discriminate between collections of 2 and 3 items, but not between 3 and 4, or between 4 and 6 (Rugani et al. 2008). Animals and infants can also perform operations such as addition and subtraction with small numerosities: babies look longer when 1 + 1 = 1 than when 1 + 1 = 2 (Wynn 1992). Dogs have similar abilities (West & Young 2002). Animals can also perform addition and subtraction on larger numbers, but then they can predict results only approximately; e. g., rhesus monkeys cannot exactly predict that 4 + 4 = 8, but they can pick out 8 when presented with possible solutions 2, 4, and 8 (Cantlon & Brannon 2007a). 0
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