Introduction During dialogue, adults tend to imitate their interlocutors’ language use. This conversational alignment occurs at multiple different levels of language and seems to function to ensure effective communication and rewarding interactions. In this study we specifically focus on the tendency for a speaker to reuse the same word as that used by a conversational partner (e.g. using brolly after your partner has used brolly; Brennan & Clark, 1996), known as lexical entrainment. It is well established that lexical entrainment implicates both unmediated processing (e.g. lexical retrieval) and mediated processing (e.g. speakers’ beliefs about their interlocutor). Unmediated theories have highlighted the role of automatic priming in lexical entrainment, whereby exposure to a partner’s lexical label (e.g. brolly) makes its representation more accessible in memory and easier to retrieve and reuse (Pickering & Garrod, 2004). Moreover, mediated theories have suggested that speakers’ beliefs may also influence their tendency to entrain – they may adapt their use of language based on what they believe a conversational partner will understand (audience design mechanisms; Clark, 1996), or in order to express affiliation and establish rapport (social-affective mechanisms; van Baaren et al., 2003). Previous studies have examined the mechanisms underlying lexical entrainment using group-level comparisons (e.g. Branigan et al., 2011). However, mediated and unmediated processing mechanisms can also vary as a result of individual differences (Tobar-Henriquez et al., 2019); this thus provides the opportunity to understand how and why lexical entrainment might vary based on individual characteristics such as age, prosociality and language ability. However, in order to understand the role of individual differences in lexical entrainment, we must first establish whether lexical entrainment is a trait which is stable within individuals. We have previously established this in adults aged 18-60 (Tobar-Henríquez et al., 2020) and in the following age groups: 18-39, 40-59, and over 60 (Tobar-Henríquez & Wilks, 2021). Furthermore, previous studies have found that lexical entrainment in adulthood increases with age, so that older speakers entrain more than younger speakers (Tobar-Henríquez, 2020; Tobar-Henríquez & Wilks, 2021). This is an important finding because it suggests that lexical entrainment mechanisms change across adulthood, thus opening up new directions to investigate both lexical entrainment mechanisms, in particular, and language processing across the lifespan, in general. One possible explanation for our finding of increased lexical entrainment with age (Tobar-Henríquez, 2020; Tobar-Henríquez & Wilks, 2021) is that priming mechanisms underlying entrainment are relied upon more heavily as speakers age. Older adults find retrieving words from memory (lexical retrieval) more difficult than younger adults. For example, they experience more tip-of-the-tongue states, take longer to find the right words (e.g. ‘dog’) for target objects (e.g. a dog), and make more naming mistakes than younger adults, using more semantically-related but inaccurate words for targets (e.g. referring to a dog as ‘cat’; e.g. Burke & Laver, 1990; Mitchell, 1989; Nicholas et al., 1985; Thomas et al., 1977). However, their difficulties seem restricted to accessing the phonological, rather than the semantic, representations of words. This is evidenced by the fact that elderly speakers make correct prime-target semantic associations (e.g. associating dog with cat) and successfully use phonological cues to overcome lexical retrieval difficulties (e.g. successfully recalling dog after being told that the word started with the letter d; Barresi et al., 2000; Juncos-Rabadán et al., 2009; Kavé & Mashal, 2012; Nicholas et al., 1985). Thus, older speakers should find retrieving a name, that they have not recently processed, more difficult than younger speakers. This may increase the likelihood that they will entrain to the specific term used by their partner as the term has been processed more recently and thus is easier to retrieve, resulting in increased entrainment with age. Yet, it is not known how age may affect whether such an entrained term is later re-used with a new partner. Moving forward, we believe this “maintenance of entrained terms” to be an important consideration because in order to communicate inter-generationally an individual needs to continuously update their language to incorporate changes driven by younger speakers. Lexical entrainment could thus represent a means by which to learn new lexical labels and the correct contexts of use (Tobar-Henríquez et al., 2021). The extent to which entrained terms are maintained may result from an interplay between lexical priming mechanisms and perspective-taking abilities. Perspective-taking decreases with age (Horton & Spieler, 2007) but reports on lexical priming mechanisms and age are somewhat mixed, with most researchers failing to find an age effect (Barry et al., 2006; Jones & Estes, 2012; Mitchell, 1989; Mitchell et al., 1990; Stern et al., 1991). Additionally, the studies often fail to replicate due to limited participant numbers and a focus on reaction times. Therefore, assuming reduced perspective-taking abilities and intact priming mechanisms, we might expect older adults to maintain use of terms used with one partner, with a new partner, at higher levels than younger adults. This is because understanding that a new partner doesn’t necessarily favour the entrained term, and modulation of priming effects driving entrainment, may require perspective taking. That is, lexical priming mechanisms could be activated by default but potentially over-ridden in certain situations in order to enable more flexible communication. Older adults may therefore display maladaptive over-maintenance of entrained terms (i.e. maintenance that might lead to miscommunication; see converging evidence from young children’s referential language use; Garrod & Clark, 1993). Social cooperativeness increases with healthy aging such that older people tend to be more interested than younger people in engaging in emotionally meaningful activities/increasing rapport during social interaction. They are also more cooperative and emotionally empathetic than younger adults (Beadle et al., 2015; Blanchard-Fields, 2007; Carstensen et al., 1995). It is therefore possible that older adults might entrain more often than younger adults in order to reduce social distance and increase rapport with their conversational partner. This would implicate prosocial accounts of lexical entrainment. In this case, we would expect there to be a positive association between individual prosocial tendencies and lexical entrainment such that adults who are more prosocial entrain more often. We have previously noted that, in order to understand age differences in lexical entrainment, it was important to establish the stability of lexical entrainment in young, middle-aged and older adults (Tobar-Henríquez & Wilks, 2021). In a similar vein, if we want to understand age differences in the maintenance of lexically entrained terms, we must first establish whether this maintenance is stable in young, middle-aged and older adults. In the current study, we will therefore first confirm that lexical entrainment (aiming to replicate our previous findings), and maintenance of lexical entrainment (an extension to these findings), are stable traits within three groups of participants aged 18-39, 40-59 and over 60. We will then investigate how age might affect lexical entrainment (aiming to replicate our previous findings) and maintenance of lexical entrainment (an extension to these findings). Additionally, we will perform an exploratory investigation into the effect of prosociality on lexical entrainment by examining whether individual differences in prosociality predict lexical entrainment. In addition to our lexical entrainment instrument, we thus use an instrument which is able to consistently distinguish between individuals’ levels of prosociality (see “The present experiment” section).