Manoochehri, Majid. 2019. “The Evolution of Memory Span: A Review of the Existing Evidence.” PsyArXiv. March 27. doi:10.31234/osf.io/zdsya
Abstract: Memory span in humans has been intensely studied for more than a century. In spite of the critical role of memory span in our cognitive system, which intensifies the importance of fundamental determinants of its evolution, few studies have investigated it by taking an evolutionary approach. Overall, we know hardly anything about the evolution of memory components. In the current study, I reviewed the experimental studies of memory span in humans and non-human animals and the evolutionary hypotheses.
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Memory span refers to the ability of an individual to reproduce immediately, after one presentation, a series of discrete stimuli (e.g., digits, letters, and words) in their original order (Blankenship, 1938). Hermann Ebbinghaus (1850-1909) has been known as one of the first researchers of memory span. He studied the number of trials that it took him to memorize sequences of nonsense syllables (Ebbinghaus, 1885). He found that the number of trials needed increased with the length of the sequences. He then commented that: “The question can be asked: What number of syllables can be correctly recited after only one reading? For me the number is usually seven” (p. 36). In the years after that, several studies of memory span with verity of methods, tasks, and materials have been done by others (for review see Blankenship, 1938; Dempster, 1981). Surprisingly, some parts of results, such as the average memory span of adults were almost always the same. In 1956, Miller in his article discussed this issue. He stipulated that: “Everybody knows that there is a finite span of immediate memory and that for a lot of different kinds of test materials this span is about seven items in length” (p. 11). Today Miller’s suggestion has been known as the “magical number seven” of Miller, which refers to the memory span of young English adults. However, further findings from other nations were also in accordance with these results, suggesting that a memory span of about seven items is a universal characteristic of human beings. In addition to this feature, memory span scores have been discussed to be invariable and resistant to Flynn effects (Gignac, 2015; Wechsler, 1939). Moreover, memory span scores do not show considerable sex differences (Lynn & Irwing, 2008).The question that arises here is which selection pressures or evolutionary events caused the current memory span of humans and its particular characteristics.
The memory span of non-human animals
Firstly, it should be considered that because of the particular difficulties of studying cognitive functions in non-human animals, designed tasks in most of the available studies are somehow easier than classical memory span tests in humans. They usually include recognition of items instead of imitating the sequences, or several hours practice before the main test.
Chimps and other primates
Few studies have investigated memory span in non-human animals. Chimpanzees have been known as one of our nearest primate relatives and also one of the smartest non-human animals. Therefore, it may be expected to find the largest memory span of non-humans in them. The existing evidence shows that chimpanzees have a memory span of about 5 items (Inoue & Matsuzawa, 2007; Kawai & Matsuzawa, 2000). The results of other primates are very close to that of chimpanzees. Studies in baboons reveal a memory span of about 4 to 5 items (Fagot & De Lillo, 2011). On the other hand, the results of studying two rhesus monkeys by Swartz et al. (1991) suggest a memory span of about 4 objects.
Non-primates
Studies of non-primates do not exhibit better performances. Herman et al. (2013) have suggested a memory span of about 4 to 5 items for bottlenose dolphins. Lately, Toyoshima et al. (2018) have stipulated that rats are able to remember 5 objects at once. By contrast, a previous similar work by Sugita et al. (2015) has argued that rats’ memory span is approximately 4 items. A memory span of 4 items has also been suggested for pigeons. Terrace (1993) has discussed that the amount of time it takes a pigeon to learn a 4-item list (i.e., 3-4 months) suggests that 4 items may approach the limit of the pigeon's memory span.
Taking all these data together, one may conclude that there is an increasing trend of memory spans with a gentle slope from other animals to us. Moreover, it also seems reasonable to argue that while the memory span of most of the mentioned animals is about 4 to 5 items, humans’ memory span is about 7 items, which implies a sudden increase. This can be interpreted in favor of Coolidge and Wynn's (2005) hypothesis. They proposed that a mutation increased the length of memory span in the relatively recent human past. No need to emphasize that the idea of an increase with a gentle slope or a sudden increase is based on only a few experiments of memory span in some non-human animals. Surely, more studies are needed to provide the exact pattern. Experimental studies are also essential to measure the memory span of the current hunter-gatherers societies, such as Hadza of Tanzania. There are several reasons for the importance of their memory span results. Namely, because they are still foragers and their lifestyle is very close to that of our hunter-gatherer ancestors. Accordingly, their memory span might be similar, too. In addition, they are not literate or numerate and do not live in information-based societies. Therefore, they are not affected as broadly as other societies by some elements such as advanced educational programs. Moreover, another question is whether they apply memory’s strategies similar to people from other societies or not.
Evolutionary discussions and hypotheses
Until today, few studies have discussed memory span from an evolutionary perspective. There are also scant studies that discussed the evolution of working memory (e.g., Carruthers, 2013). It should be noticed that there are considerable differences between the functions of memory span and working memory. Therefore, there might be different selection pressures on them. In the present study the primary focus is on memory span.
Given the question of why memory span of humans has such a limited capacity, MacGregor (1987) based on a mathematical discussion emphasized the importance of an efficient retrieval. He suggested that: “… in a memory system evolved for efficient retrieval, there is an upper effective limit to short-term memory” (p. 107). From his point of view, there is an upper effective limit for the number of items in memory span, and this limitation guarantees an efficient retrieval.
The evolution of memory span has been also discussed by Coolidge and Wynn (2005). Although they mainly focused on working memory, the phonological lop which refers to memory span (or verbal memory span), has also been argued in their study. The core suggestion of their article is that a genetic mutation affected neural networks approximately 60,000 to 130,000 years ago and increased the capacity of general working memory or phonological storage. In case of memory span, they stipulated that:” A relatively simple mutation that increased the length of phonological storage would ultimately affect general working-memory capacity and language” (p. 14). They emphasized that an enhancement of capacities occurred in the relatively recent human past, most likely after the first appearance of anatomically modern humans, and this development was the final piece in the evolution of human executive reasoning ability, language, and culture. From their point of view, the larger capacity is a necessary precondition for symbolic thought, which selective pressures contributed to the growth of it. They noted that an increase in memory span of pre-modern Homo sapiens would have allowed greater articulatory rehearsal, consequently allowing for automatic long-term storage, and the beginnings of introspection, self-reflection, and consciousness. They, provided some evidence to support the assumption of a genetic mutation, such as the beginnings of composite tool-making about 300,000 years ago, or an explosion of culture which began approximately 50,000 years ago.
In another part of their article, Coolidge and Wynn (2005) have quoted from Alan Baddeley that: “the phonological store evolved principally for the demands and acquisition of language” (p. 9). In addition, they have proposed that, evolutionary, the visuospatial sketchpad which is a part of working memory and maintains visual and spatial information may be older than the phonological loop. It should be noticed that the visuospatial sketchpad refers to visual memory span.
Conclusions
In the first place, the present article draws attention to the gap of evolutionary studies of memory span. It also suggests an increasing trend of memory span from our ancestors to us, whether the trend includes a steady increase with a gentle slope or a sudden increase. In terms of evolutionary discussions, the few available studies which have argued the evolution of memory span concentrated on some particular issues such as the reasons of a limited-capacity construct, or the results of increasing the length on cognitive functions. Other aspects remain untouched.
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